Chu Yan, Morovich, and Guan Yifan were floating through a long corridor on Blue Space. In front of them drifted an information window showing a map of the interior of the ship. Three red dots flashed on the map, and the three of them were approaching one of these dots now.

"Right there!" Guan pointed straight ahead.

"It's possible to detect the warped points using the naked eye," Chu Yan said. "But the best way is to monitor the electromagnetic radiation. The emission from these points is very faint, but it has a distinct spectral signature. The regular sensors on our ships can detect and locate them. Typically, a volume of space as large as a ship in this region contains one to two warped points, but we once found twelve at once. Look, right now there are three."

In the bulkhead in front of them was a circular hole about a meter in diameter. The edge was smooth, mirrorlike. Through the hole they could see tubes of various thicknesses. Several tubes had entire sections missing in the middle. In two of the thicker tubes, they could see liquid flowing. The liquid seemed to flow from one section, disappear, and then reappear in the corresponding section of the tube on the other side. The missing sections were of different lengths, but overall seemed to describe a spherical space. Based on the shape of the missing sections, part of the invisible bubble protruded into the corridor. Morovich and Guan carefully avoided it.

"Let's go in," said Chu Yan. Then he jumped into the bubble, as though diving into a pool. Morovich and Guan watched in consternation as Captain Chu disappeared from head to toe. The cross section of his body on the surface of the invisible bubble rapidly changed shape, and the mirrorlike edge of the hole threw reflections onto the surrounding bulkheads like ripples.

As Morovich and Guan stared at each other, two hands and forearms emerged from the bubble and remained suspended in midair. Each hand grabbed hold of one of theirs, and they were pulled into fourth-dimensional space.

Chu Yan carelessly extended a hand into the invisible bubble, and half of his arm disappeared. Standing to the side, Guan Yifan saw a clear cross section of his cut-off arm, like the cross sections of Vera's legs that had been seen by Sublieutenant Ike aboard the Gravity. Chu Yan pulled his arm back and showed the astonished Morovich and Guan that it was unharmed. Then he encouraged them to try it. The two carefully reached into the invisible bubble. They watched their hands, and then arms, vanish, but they felt nothing.

All who had experienced it agree that the sensation of being in four-dimensional space was indescribable. They would even say that it was the only thing encountered by humanity thus far that absolutely could not be captured by language.

People usually resorted to this analogy: Imagine a race of flat beings living inside a two-dimensional picture. No matter how rich or colorful the picture was, the flat people could only see the profile of the world around them. In their eyes, everything consisted of line segments of various lengths. Only when such a two-dimensional being was taken up out of the picture into three-dimensional space and looking down on the world could he see the entirety of the image.

This analogy simply expressed in some more detail the indescribability of experiencing four-dimensional space.

A person looking back upon the three-dimensional world from four-dimensional space for the first time realized this right away: He had never seen the world while he was in it. If the three-dimensional world were likened to a picture, all he had seen before was just a narrow view from the side: a line. Only from four-dimensional space could he see the picture as a whole. He would describe it this way: Nothing blocked whatever was placed behind it. Even the interiors of sealed spaces were laid open. This seemed a simple change, but when the world was displayed this way, the visual effect was utterly stunning. When all barriers and concealments were stripped away, and everything was exposed, the amount of information entering the viewer's eyes was hundreds of millions times greater than when he was in three-dimensional space. The brain could not even process so much information right away.

Given this description, those who had never experienced four-dimensional space might get the wrong impression that they were seeing everything "through" the hull. But no, they were not seeing "through" anything. Everything was laid out in the open, just like when we look at a circle drawn on a piece of paper, we can see the inside of the circle without looking "through" anything. This kind of openness extended to every level, and the hardest part was describing how it applied to solid objects. One could see the interior of solids, such as the bulkheads or a piece of metal or a rock -- one could see all the cross sections at once! Morovich and Guan were drowning in a sea of information -- all the details of the universe were gathered around them and fighting for their attention in vivid colors.

In Morovich and Guan's eyes, Blue Space was a magnificent, immense painting that had just been unrolled. They could see all the way to the stern, and all the way to the bow; they could see the inside of every cabin and every sealed container in the ship; they could see the liquid flowing through the maze of tubes, and the fiery ball of fusion in the reactor at the stern… Of course, the rules of perspective remained in operation, and objects far away appeared indistinct, but everything was visible.

Morovich and Guan had to learn to deal with an entirely novel visual phenomenon: unlimited details. In three-dimensional space, the human visual system dealt with limited details. No matter how complicated the environment or the object, the visible elements were limited. Given enough time, it was always possible to take in most of the details one by one. But when one viewed the three-dimensional world from four-dimensional space, all concealed and hidden details were revealed simultaneously, since three-dimensional objects were laid open at every level. Take a sealed container as an example: One could see not only what was inside, but also the interiors of the objects inside. This boundless disclosure and exposure led to the unlimited details on display.

Everything in the ship lay exposed before Morovich and Guan, but even when observing some specific object, such as a cup or a pen, they saw infinite details, and the information received by their visual systems was incalculable. Even a lifetime would not be enough to take in the shape of any one of these objects in four-dimensional space. When an object was revealed at all levels in four-dimensional space, it created in the viewer a vertigo-inducing sensation of depth, like a set of Russian nesting dolls that went on without end. Bounded in a nutshell but counting oneself a king of infinite space was no longer merely a metaphor.

Morovich and Guan looked at each other, then looked at Chu Yan to the side. They saw bodies revealed at every level with the details displayed in parallel: They could see bones, organs, the marrow inside the bones, the blood flowing through the ventricles and atria of the heart, the openings and closings of the mitral and tricuspid valves. When gazing at each other, they could see clearly the interior structure of the lenses of the eyes…

The phrase "in parallel" might also lead to misunderstanding. The physical locations of the parts of the body had not been shifted around: The skin still enclosed the organs and the bones, and the familiar shape of each person in three-dimensional space persisted -- but it was now merely one detail among an infinity of details, visible at the same time, in parallel.

"Be careful of where you move your hands," Chu Yan said. "You might poke an internal organ -- yours or someone else's -- by mistake. But as long as you don't use too much force, even contact won't be a big deal. You'll feel a bit of pain or nausea, and of course there's the risk of infections. Also, don't touch or move things unless you know exactly what they are. Everything on the ship is now naked: You might be touching a live wire or heated steam, or even integrated circuits, and cause system malfunctions. Overall, you're like gods in the three-dimensional world, but you have to get used to being in four-dimensional space before you can use your powers effectively."

Morovich and Guan quickly learned how to avoid touching inner organs. By moving in a certain direction, they could grasp someone's hand instead of the bones inside the hand. To touch the bones or organs required exertion in another direction -- a direction that didn't exist in three-dimensional space.

Next, Morovich and Guan discovered something else that excited them: They could see the stars in every direction. They could see the bright glow of the Milky Way extending through the universal eternal night. They knew that they were still inside the ship -- none of them wore space suits, and all breathed the air on the ship -- but in the fourth dimension, they were exposed to space. All three veteran spacers had performed countless spacewalks, but they had never felt so utterly intimate with space. During spacewalks, they were at least enclosed by space suits, but now, nothing stood between space and them. The ship that revealed infinite details did not shield them from space. In the fourth dimension, the ship lay in parallel with all of space.

The brain that had been adapted from birth to sense and feel three-dimensional space could not handle the infinite information generated by countless details, and initially, information overload threatened to shut down processing. But the brain soon grew used to the four-dimensional environment, and without conscious thought it learned to ignore most details, leaving only the frames around objects.

After the initial vertigo, Morovich and Guan experienced another, even greater shock. Once their attention was no longer completely absorbed by the inexhaustible details of the environment around them, they sensed space itself, or sensed the fourth dimension. Later, people would call this feeling "high-dimensional spatial sense." For those who had experienced it, high-dimensional spatial sense was hard to convey in words. They often tried to explain it this way: Concepts like "vastness" or "boundlessness" in three-dimensional space were replicated an infinite number of times in four-dimensional space in a direction that did not exist in three-dimensional space. They often resorted to the analogy of two mirrors facing each other: In either mirror one could see a boundless multitude of replicated mirrors, a hall of mirrors that extended into infinity. In this analogy, each mirror in the hall was a three-dimensional space. In other words, the vastness that one experienced in three-dimensional space was but a cross-section of the vastness of four-dimensional space. The difficulty of describing high-dimensional spatial sense lay in the fact that for observers situated in four-dimensional space, the space they could see was empty and uniform, but there was a depth to it that could not be captured by language. This depth was not a matter of distance: It was bound up in every point in space. Guan Yifan's exclamation later became a classic quote: "A bottomless abyss exists in every inch."

"How does one find a warped point in four-dimensional space?" Morovich asked.

Chu Yan said, "We should return. The warped points remain stable for only a brief period of time before drifting away or disappearing. To find a new warped point requires moving in four-dimensional space. That's a dangerous undertaking for novices like you."

The experience of high-dimensional spatial sense was a spiritual baptism. In one moment, concepts like freedom, openness, profundity, and infinity all gained brand-new meanings.

Chu Yan pointed in the direction they had come from. Morovich and Guan saw those tubes again, which now also lay open so that all the liquids flowing through them could be seen clearly. Inside the spherical region, the tubes were curved and distorted, and the sphere appeared like a drop of dew hanging on a spiderweb. This was different from how the region had appeared in three-dimensional space. There, the warped point didn't refract light, and so appeared completely invisible. Its presence could only be made known via the disappearance of objects that had entered four-dimensional space inside the bubble.

"Simple. A warped point is usually spherical. Light is refracted inside the sphere, and objects inside are distorted, causing a visual break in the image of the objects. Of course, this is just an optical effect in four-dimensional space, not a real change in the shapes of the objects. Look over there --"

Chu Yan indicated that the other two should follow, and entered the dewdrop-like bubble. In an instant they were back inside the three-dimensional world, back in the corridor inside the ship, in exactly the place where, ten minutes ago, they had entered four-dimensional space. In fact, they had never left -- the space they were in had just gained an extra dimension. The round opening in the bulkhead was still there, and they could still see the "broken" tubes inside.

"If you come here again, you must wear space suits. Novices can't always tell locations accurately, and finding a new warped point for the return trip might result in landing outside the ship in three-dimensional space."

But to Morovich and Guan, the world no longer seemed familiar. They now experienced the three-dimensional world as narrow and smothering. Guan dealt with it slightly better -- he had at least experienced four-dimensional space once before, in a semiconscious state. But Morovich was feeling claustrophobic, as though he was being suffocated.

"How did all this happen?" Morovich tore open his collar and gasped.

"This is normal. You'll get used to it after a few times." Chu Yan laughed. "The two of you now know what real vastness means. Even if you put on space suits and go take a walk in space now, you'll feel confined."

"We entered a region of space where space has four dimensions. That's it. We call this region a four-dimensional fragment."

"Four-dimensional space contains three-dimensional space, just as three-dimensional space contains two-dimensional space. To make another analogy: We're located inside a three-dimensional sheet of paper in four-dimensional space."

"Let me propose a model," said Guan excitedly. "The entirety of our three-dimensional space is a large, thin sheet of paper, sixteen billion light-years across. Somewhere on this sheet of paper is a tiny, four-dimensional soap bubble."

"But we're in three-dimensional space now!"

"Perfect, Dr. Guan!" Chu Yan clapped Guan's shoulder, making him tumble in weightlessness. "I've been trying to come up with a good analogy, and you hit it in one try. This is why we need a cosmologist! You're exactly right. We were on this three-dimensional sheet of paper, crawling over the surface. Then we came into the soap bubble. From a warped point, we can manage to leave the surface of the paper and entered the space inside the bubble."

"Though we were in four-dimensional space just now, our bodies remained three-dimensional," Morovich said.

"What exactly are warped points?"

"The three-dimensional sheet of paper is not completely flat everywhere. Some places are warped, reaching into the fourth dimension. That's what a warped point is: a tunnel from lower dimensions into higher dimensions. We can get into four-dimensional space by jumping into them."

"Are there many warped points?"

"That's right. We were flat, three-dimensional people drifting in four-dimensional space. It's not clear how our bodies are able to survive in four-dimensional space, given that the physical laws are likely different. There are many such mysteries."

"Oh yes. They're everywhere. Blue Space was able to discover their secret earlier because we have more people aboard, and so there were many more opportunities to encounter warped points. Gravity, besides having fewer people, also had a much stricter psychological monitoring regime -- even those who encountered warped points dared not discuss them."

"Are all warped points this small?"

"No. Some are much bigger. There's one mystery we've never been able to solve: We once observed that the rear third of Gravity had warped into four-dimensional space, and stayed there for several minutes. How could you have not noticed anything odd?"

"Well, the last third of the ship usually had no crew. Oh, wait, he was there." Morovich turned to Guan Yifan. "You must have experienced that. I think I heard about it from Dr. West."

"I was only half awake. Later, that idiot convinced me that I was only hallucinating."

"It's not possible to look into the fourth dimension from three-dimensional space. But it is possible to be in four-dimensional space and see everything happening in three-dimensional space and to affect things in it. We were able to lay an ambush against the droplets from four-dimensional space. No matter how powerful the strong-interaction probes were, they were still three-dimensional objects. In a sense, three-dimensionality is synonymous with fragility. Viewed from four-dimensional space, it was an unrolled painting, defenseless. We approached it from the fourth dimension, and, without understanding its principles of operation, sabotaged its internal mechanisms -- completely exposed -- at random.

The round opening in the bulkhead next to the three of them began to move slowly and to shrink. When it was just over ten meters from them, the opening disappeared completely. But the information window floating near them indicated that two more warped points had appeared aboard Blue Space.

"We think not."

"Is Trisolaris aware of the existence of the four-dimensional fragment?"

"How could a four-dimensional fragment appear in three-dimensional space?" Guan Yifan muttered to himself.

"About the size of the Solar System."

"Talking about the size of four-dimensional space from three-dimensional space is kind of meaningless. We can only discuss how big the projection of the fragment is in three dimensions. Based on preliminary investigation, we think the three-dimensional projection is spherical. If that's true, based on the data collected so far, its radius is between forty to fifty AU."

"No one knows. Doctor, this is your puzzle to solve."

"The soap bubble -- uh, four-dimensional fragment -- how big is it?"

After the discovery of the four-dimensional fragment, Blue Space had explored and studied the space inside extensively. The addition of Gravity brought more advanced equipment and techniques, so the crews could conduct more comprehensive and in-depth exploration.

Through the telescope, they discovered a ring-shaped object. Since it was impossible to determine its distance from the ship, it was also impossible to estimate its size. The best guess was that its three-dimensional diameter was between eighty to one hundred kilometers, and the band of the ring was about twenty kilometers thick. The whole thing resembled a giant wedding band spinning in space. Complex patterns resembling circuitry could be discerned on the surface of the band. Based on the evidence, it seemed reasonable to conclude that the ring had been constructed by intelligent beings.

In three-dimensional space, this region appeared very empty and showed no irregularities. Much of the exploration had to be conducted in four-dimensional space. Since releasing probes into four-dimensional space was no trivial matter, most of the research was conducted by inserting a telescope into the fragment through a warped point. Manipulating a three-dimensional instrument in four dimensions required some practice, and a period of adjustment, but once the scientists got the hang of it, they immediately made shocking discoveries.

But the most shocking thing was that the "Ring" was sealed. It existed in four-dimensional space, but didn't reveal its interior as a three-dimensional object would have. Since its inside was concealed, that meant it was a true four-dimensional object. This was also the first true four-dimensional object detected by humanity since entering four-dimensional space.

People initially feared an attack, but the surface of the Ring showed no signs of any activity. They also detected no emission of any electromagnetic, neutrino, or gravitational wave signals. Other than a slow, stately revolving motion, the Ring showed no acceleration. The working theory was that this was a ruin, perhaps a long-abandoned space city or spaceship.

This was the first time humanity had observed another civilization outside of the Earth and Trisolaris.

Further observations revealed more unknown objects in the depths of four-dimensional space. They were all sealed four-dimensional objects of different sizes and shapes, and many seemed to be artifacts fashioned by intelligence: pyramids, crosses, polyhedral frames, and so on. Other objects were irregular forms composed from simpler shapes, also clearly not natural. More than a dozen of these objects had shapes that could be discerned by telescope, though farther away were many more objects that appeared only as point sources. In total, about a hundred such objects were found. Like the Ring, none of them showed signs of activity, and they emitted no detectable signals.

Similarly, locating a warped point to return to three-dimensional space would be very difficult. Since the coordinate in one of the four directions could not be determined, when a warped point was found, all that was known was its direction, not its distance from the observer. The crew of the pinnace could use a warped point to return to three-dimensional space and, to their surprise, find themselves far away from Blue Space.

Guan Yifan proposed to Captain Chu the plan of piloting a pinnace to the Ring and studying it up close. If possible, he wanted to enter the Ring. The captain denied this proposal unequivocally. Navigating through four-dimensional space was fraught with risks. To precisely fix one's location required four coordinates, but equipment brought from three-dimensional space could only determine three coordinates. This meant three-dimensional explorers could not determine the location of any object in four-dimensional space with precision. An explorer could not ascertain the location or distance of the Ring using instrumentation or visual observation, so it would be possible to collide with the Ring at any moment.

After that, Blue Space and Gravity suffered six micrometeoroid strikes in one day. A 140-nanometer micrometeoroid struck and completely destroyed the magnetic levitation controller of Blue Space's fusion reactor core. This was a key system aboard the ship. The fusion reactor core could reach temperatures as high as a million degrees, which would vaporize any material it came in contact with. A magnetic field kept it centered within the reaction chamber. If the controller failed, the superheated reactor core could escape from the magnetic field and instantly destroy the ship. Fortunately, the backup unit kicked in immediately and shut off the reactor -- which was operating at minimal power -- and averted catastrophe.

Finally, most of the radio waves linking Blue Space and the pinnace would spill into the fourth dimension, leading to far faster decay of signal strength and causing communication difficulties.

As the two ships sailed deeper into the four-dimensional fragment, the density of micrometeoroid strikes increased, and even larger meteoroids, visible to the naked eye, passed near the ships. Their velocity relative to the ships was several times the third cosmic velocity. In three-dimensional space, the critical parts of the ships were wrapped in layers of protection, but here, they lay exposed to the fourth dimension, completely defenseless.

"Even if you focus only on the practical, we can probably recover unimaginable knowledge and artifacts from that ring-shaped ruin."

Guan Yifan raged against this decision. "The greatest mystery in the universe is right in front of us. The answers to all our cosmological questions may be found here. How can we leave?"

"Are you talking about the three and three hundred thousand syndrome? The fragment did remind me of it."

Chu Yan decided that the two ships should retreat from the four-dimensional fragment. The fragment as a whole was moving away from the Solar System, heading in the same direction as the ships' course; thus, although Blue Space and Gravity were sailing away from the Solar System at a high speed, their velocity relative to the fragment was small, and they had only slowly caught up to the fragment. They weren't deep within the fragment and should be able to decelerate and leave it easily.

"Such gains are only meaningful if we survive this ordeal. Right now, both of our ships could be annihilated any moment."

Guan sighed and shook his head. "Fine. But before you leave, let me ride a pinnace to explore the Ring. Give me a chance. You spoke of survival, but perhaps our future survival depends on what I can discover here!"

"In a four-dimensional world, only a live observer can understand what is seen. You know this better than I."

"We can consider sending a drone."

Prior to this, the longest voyage in four-dimensional space had been the attack on the droplets and Gravity. During the attack, a pinnace had sailed through four-dimensional space to approach Gravity, and then three people, including Lieutenant Commander Park Ui-gun, had entered Gravity via a warped point to reconnoiter. Thereafter, more than sixty marines had boarded Gravity in three separate waves. The attack on the droplets had relied on smaller shuttles. But this voyage of discovery to the Ring would be far longer.

After a brief discussion, the senior staff of both ships approved Guan's proposal. Guan Yifan, Lieutenant Zhuo Wen, and Dr. West made up the exploratory team. Lieutenant Zhuo was the science officer aboard Blue Space and had comparatively extensive experience navigating in four dimensions. Dr. West, on the other hand, simply insisted that he come; the request was ultimately approved because he had studied Trisolaran language before the voyage.

The pinnace entered four-dimensional space from a warped point between the two ships. At the tail of the pinnace, the small fusion reactor's core turned from dim red to a faint blue as its power level increased. This flame, together with the balls of fire in the reactors of the two larger ships, illuminated this world of infinity times infinity. Blue Space and Gravity quickly receded, and as the pinnace sailed deeper into four-dimensional space, the high-dimensional spatial sensation intensified. Although Dr. West had already been to four-dimensional space twice, he exclaimed, "How grand is the spirit that can grasp such a world!"

Lieutenant Zhuo piloted the pinnace using voice commands or by moving the cursor with his gaze -- it was a good idea to avoid using hands and risking contact with some sensitive piece of equipment that now lay exposed in four dimensions. To the naked eye, the Ring was still but a barely visible dot, but Zhuo cautiously kept the pinnace flying at a very low speed. Due to the extra unmeasurable dimension, visual judgments of distance were completely unreliable. The Ring might be as far away as an astronomical unit or as close as the pinnace's bow.

All of a sudden, the coin-sized Ring turned into a gigantic structure. Lieutenant Zhuo banked sharply and barely managed to avoid collision. The pinnace passed through the Ring like an arch in space. The pinnace decelerated, turned around, and came to a stop a short distance from the Ring.

After three hours, the pinnace had already exceeded the previous record of distance sailed from the ships in four-dimensional space. The Ring remained but a dot. Lieutenant Zhuo grew even more cautious and was prepared to decelerate at full power and to change the heading at a moment's notice. Guan Yifan grew impatient and asked Zhuo to fly faster. Just then, West cried out in surprise.

"You've got to remember that we're basically blind in the fourth dimension," Lieutenant Zhuo said. He decreased their speed again.

The Ring turned into a real ring -- it just happened. One moment, it was still a dot; the next moment, it was a ring the size of a coin. There was no gradual process of change at all.

Two more hours passed. If they were still in three-dimensional space, they would have sailed about two hundred thousand kilometers.

From up close, the surface of the Ring appeared very different from images taken by the telescope. Instead of a golden yellow light, it gave off a dark copper glow. Those faint etched lines that had looked like circuitry were really lines left by micrometeoroids striking its surface. There was still no evidence of any activity, and it didn't emit light or other radiation. Looking at the ancient surface of the Ring, all three felt a sense of familiarity. They recalled the destroyed droplets, and then tried to imagine the immense four-dimensional Ring with a mirrorlike smooth surface -- it would have been a breathtaking sight.

This was the first time humans had come close to a four-dimensional object. Similar to high-dimensional spatial sense, they felt the magnificence of high-dimensional materiality. The Ring was completely sealed, and they could not look inside the band, but they could feel an immense sense of depth and of containment. What they were seeing wasn't just a Ring, but an infinity of Rings all stacked together in concealment. This sensation of four-dimensionality impressed itself upon the soul, and gave the observers the experience of seeing the mountain contained in a mustard seed described in Buddhist parables.

The original plan had intended the hailing message as an experiment; there was no preparation for how to develop further communications. While the three in the pinnace debated what to do, the Ring sent a second bitmap to the pinnace: 1, 3, 5, 7, 11, 13, 1, 4, 2, 1, 5, 9.

Then a third bitmap: 1, 3, 5, 7, 11, 13, 16, 6, 10, 10, 4, 7.

They weren't expecting any answers, but an answer arrived immediately, so fast that they couldn't believe their eyes. The information window hovering in the middle of the pinnace cabin displayed a simple bitmap similar to the one they had sent. It also consisted of six lines portraying the next six prime numbers: 17, 19, 23, 29, 31, 37.

Following the preestablished plan, Lieutenant Zhuo transmitted a message to the Ring via medium-frequency radio waves. This was a simple bitmap, a bit array that could be interpreted as six lines of dots that formed a sequence of prime numbers: 2, 3, 5, 7, 11, 13.

A fifth: 1, 3, 5, 7, 11, 13, 7, 2, 16, 4, 1, 14.

A fourth: 1, 3, 5, 7, 11, 13, 19, 5, 1, 15, 4, 8.

The bitmaps came one after another. The first six numbers in each consisted of the six prime numbers sent by the pinnace as a greeting. As for the next six numbers in each series, both Lieutenant Zhuo and Dr. West turned to Guan Yifan, the scientist. The cosmologist stared at the scrolling numbers in the floating window and shrugged.

"They -- or it -- want to know everything about us?"

"We have to ask for authorization."

"I can't see any pattern."

"Or at least a linguistic sample. It wants to decode it, study it, and then communicate with us further."

"Then we should send it the Rosetta System."

"Then let's suppose that there is no pattern." West pointed at the window. "The first six numbers were sent by us, so it's possible that they mean 'you.' The next six numbers in the transmissions show no recognizable pattern, so maybe they mean 'all'--'everything about you.'"

The Rosetta System was a database developed to teach Earth languages to Trisolarans. The database included about two million characters' worth of documents concerning the natural and human histories of Earth with numerous videos and images. There was also software to draw connections between the linguistic symbols and the images so that an alien civilization could decode and study Earth languages.

The mother ship authorized the request from the exploration team. But the pinnace didn't have the Rosetta System in its onboard computer memory, and due to the extremely tenuous communication link between the pinnace and the mother ship, it was impossible to transmit such a large volume of data. The only solution was to have the mother ship beam the information directly at the Ring. This couldn't be done via radio, but luckily, Gravity was equipped with a neutrino communication system. They weren't sure if the Ring could receive neutrino signals, however.

Three minutes after Gravity transmitted the Rosetta System via a neutrino beam to the Ring, the pinnace received a new series of bitmaps from the Ring. The first one was a perfect square of sixty-four dots arranged eight by eight; the second bitmap was missing one dot in a corner, leaving sixty-three; the third bitmap was missing two, leaving sixty-two…

"It's a countdown, or a progress bar," said West. "I think this is to show that it has received the Rosetta System and is in the process of decoding it. We should wait."

"Why sixty-four dots?"

"It's a reasonably big number if you're in base two. It's like how we use one hundred for lots of things in base ten."

The Rosetta System was written in a language that mixed English with Chinese. It would make sense that the Ring would use the same language to communicate with them. It just happened that this message consisted entirely of Chinese characters. Guan Yifan typed a question into the floating window, and began the conversation between humanity and the Ring.

Lieutenant Zhuo and Guan were both glad to have West with them. The psychologist did seem to have some skills when it came to establishing communications with unknown intelligences.

When the countdown reached fifty-seven, something exciting happened: The next number didn't come in the form of a bitmap of dots. The Ring transmitted the Arabic number 56.

"Wow, fast learner," Guan said.

The number kept on decreasing by one every ten seconds or so. A few minutes later, the number reached 0. The last message consisted of four Chinese characters: I am a tomb.

It used to be a spaceship. But now it's dead, and so it's a tomb.

Is this a spaceship?

Whose tomb is this?

Who are you? Who is conversing with us?

You told me that you came from the sea. Did you build the sea?

Are you from far away? Or have you always been here?

The tomb of those who created it.

(There was no answer.)

You mean you're a ship whose crew died? In other words, you're the control system for the ship?

Most of them are tombs. The others will be tombs soon. I don't know them all.

I'm from far away; they're also from far away, from different places far away. Where?

(There was no reply to this.)

We can see many other objects in this region of space. Are they also tombs?

Did you build this four-dimensional fragment?

Are you saying that for you, or at least for your creators, this four-dimensional space is like the sea for us?

More like a puddle. The sea has gone dry.

Why are so many ships, or tombs, gathered in such a small space?

I am the tomb. It is the tomb speaking to you. I'm dead.

I'm a tomb; I'm dead; I won't attack anyone. There is no dark forest state between spaces of different dimensions. The lower-dimensional space cannot threaten the higher-dimensional space, and the resources of the lower-dimensional space are of no use to the higher-dimensional space. But the dark forest exists everywhere between those sharing the same dimensions.

The last sentence was like a thunderclap. The three inside the pinnace cabin and everyone in the distant two mother ships hearing the exchange via a faint link all shuddered.

Are all the fish here?

Dark forest… what do you mean?

Are you going to attack us?

The fish who dried the sea went onto land before they did this. They moved from one dark forest to another dark forest.

When the sea is drying, the fish have to gather into a puddle. The puddle is also drying, and all the fish are going to disappear.

The same thing you mean.

The fish responsible for drying the sea are not here.

We're sorry. What you said is really hard to understand.

Can you give us any suggestions?

I like fish. Can I have it?

Guan sat there, stunned for a few seconds, and then realized that the pinnace really did have some fish. He always carried an ecological sphere with him, about the size of a fist. Inside the glass sphere was water, a tiny fish, and some seaweed; together, everything formed a carefully designed miniature enclosed ecosystem. This was Guan's favorite possession, so he had taken it with him on this adventure. If he couldn't return, this would have accompanied him into the afterlife.

Toss it over.

How do we give it to you?

Leave this puddle immediately. You are thin pictures. You're fragile. If you stay in the puddle, you'll turn into tombs in no time… Wait, there seem to be fish on your pinnace.

The three put on the helmets for their space suits and opened the pinnace's hatch. Guan lifted the ecological sphere to his eyes. Carefully, since he was in four-dimensional space, he held the sphere by its three-dimensional edge, and gave it a final glance. From this four-dimensional perspective, every detail in the sphere was revealed, and this tiny world of life seemed even more rich, varied, and colorful. Guan swung his arm and tossed the sphere in the direction of the Ring. He watched as the small, transparent sphere disappeared in space. Then he closed the hatch and continued the conversation. Is this the only puddle in the universe?

Since they now knew their distance from the mother ship, the return trip could be covered twice as fast. In two hours, they were back in the vicinity of Blue Space, and successfully located a warped point to return home.

Gravity informed them that more micrometeoroids had struck Blue Space. An increasing number of drifting objects, including some small-scale four-dimensional objects, possibly debris from ships or other artifacts, surrounded both ships. Captain Chu ordered their immediate return. The plan to board the Ring had to be scrapped.

The explorers were treated as heroes and given a celebratory welcome -- even if their discoveries appeared to yield no practical applications for the future of the two ships.

"I go back to the analogy I made before. The probability that we managed to stumble into the only soap bubble with a diameter of forty to fifty astronomical units on the surface of a sheet of paper sixteen billion light-years across is so minuscule that it might as well be zero. I'm certain that there are other soap bubbles, probably many more."

There was no answer. After that, the Ring remained silent and responded no more to attempts at communication.

Captain Chu asked, "Dr. Guan, what do you think is the answer to the last question you asked the Ring?"

Blue Space and Gravity began to back out of the fragment. As the ships accelerated, gravity pulled them toward the ships' sterns. Guan Yifan and the science officers from both ships tried to cram as much research about the fragment as possible into the next few days and spent almost all their time in four-dimensional space. This was only in part due to the requirements of their research -- they also found the confinement and claustrophobia of being in three-dimensional space unbearable.

"I think there's an even more interesting question: Have we encountered them before? Think about the Earth: It's been careening through space for several billion years. Is it not possible that it had entered a four-dimensional fragment in the past?"

"Do you think we'll encounter more in the future?"

"Why are there bubbles at all? Why are there so many four-dimensional fragments in three-dimensional space?"

"That would have been an astonishing sight. I find it hard to imagine that humanity had experienced it… But I wonder if dinosaurs could have located warped points…"

"Captain, I think it's likely a dark secret."

"It's a great mystery."

This surprised them. Based on their calculations, they should still be cruising through the fragment for another twenty hours. Their early exit was probably due to one of two reasons: one, the fragment had sped up in a direction opposite to the ships' current heading. Two, the fragment was shrinking. The crews believed reason number two was more likely. Other than the data, they also remembered the answer from the Ring: When the sea is drying, the fish have to gather into a puddle. The puddle is also drying, and all the fish are going to disappear.

Blue Space and Gravity were outside the fragment.

On the fifth day after beginning acceleration, all those in four-dimensional space found themselves back in three-dimensional space in a flash without having passed through a warped point. The electromagnetic sensors on the two ships indicated that there were no more warped points aboard either ship.

The two ships stopped accelerating and began to decelerate at full power. Finally, they stopped at the vicinity of the boundary of the four-dimensional fragment, where it was safe.

But soon they did notice a strange and spectacular sight: From time to time, luminous lines appeared in the space before them. Those lines were very thin, and very straight on first appearance. They seemed to have no thickness to the naked eye, and stretched in length between five thousand and thirty thousand kilometers. The lines appeared suddenly. At first, they gave off a blue glow. Then the color gradually shifted to red, and the straight lines curved and broke into many pieces, until they finally vanished. Observation showed that these lines manifested at the edge of the fragment, as though a giant pen constantly marked out the boundary.

They launched an unmanned probe toward the region of space where the lines appeared, and by luck, the probe managed to observe one of the lines when it flashed into existence at close range. The probe was about a hundred kilometers away, rushing toward the line at full speed. By the time it arrived, the line had curled, broken up, and disappeared. The probe detected massive quantities of hydrogen and helium in the vicinity, and also some dust from heavy elements, mainly iron and silicon.

The edge of the four-dimensional fragment was invisible. The space before them was empty, placid like the surface of a deep pool. The sea of stars that was the Milky Way shone brightly, giving no hint that a great secret was hidden close by.

After analyzing the data, Guan and the science officers concluded that the lines were created by four-dimensional matter entering three-dimensional space. As the fragment shrank, four-dimensional matter dropped into three-dimensional space and decayed instantly. Although these bits of matter took up very little volume in four-dimensional space, their decay into three dimensions flattened the fourth dimension, causing their volume to increase greatly and expand into the form of straight lines. By their calculations, a few tens of grams of four-dimensional matter could form a line stretching almost ten thousand kilometers in three dimensions.

Based on the rate at which the boundary of the fragment was receding, in twenty days or so, the Ring would also enter three-dimensional space. The two ships decided to wait to observe such a wonder of the universe -- they had plenty of time, after all. Using the glowing decay-lines as markers, the two ships cautiously proceeded, maintaining the same speed as the receding edge of the fragment.

During the next dozen or so days, Guan Yifan was absorbed in deep thought and calculations, and the science officers engaged in vigorous debates. Finally, they reached the consensus that, based on current theoretical physics, they could not do too much theoretical analysis of the four-dimensional fragment. But the theories that had been developed in the past three centuries could at least make some predictions that were confirmed by observations: A higher dimension existing in macro form decayed toward lower dimensions as inevitably as water fell over a cliff. The decay of four-dimensional space into three dimensions was the root cause of the fragment's shrinking.

They could again see the Ring with the naked eye. The existence of this self-declared tomb would soon end in three-dimensional space.

But the lost dimension wasn't truly lost: It simply curled up from the macroscopic to the microscopic and became one of the seven dimensions folded up within the quantum realm.

Both Blue Space and Gravity stopped advancing and backed off three hundred thousand kilometers. As the Ring entered three-dimensional space, the decay process would release tremendous amounts of energy -- this was why the lines that appeared earlier had given off so much light.

Twenty-two days later, the edge of the fragment receded past the Ring. The moment the Ring entered three-dimensional space, the universe seemed to be cut in half. The cut surface glowed with a blinding light, as though a star had been pulled into a line in an instant. From the spaceships it was impossible to see the end points, but it was as if God had held a T-square against the plan for the universe and sketched a line straight across from left to right. Careful observation by instruments revealed that the line was close to one AU in length, or about 130 million kilometers, almost long enough to connect the Earth with the Sun. In contrast to other lines that had been observed, this one had a thickness that was visible even from several hundred thousand kilometers away. The light emitted by the line turned from a hot bluish-white to a merely warm red, and then gradually dimmed. The line itself also became twisted and loose, breaking into a belt of dust. It no longer glowed by itself, but seemed suffused with the light of the stars, a serene silver. Observers from both ships shared a strange impression: The dust belt resembled the Milky Way in the background. What had happened seemed to be the flash of a giant camera that took a picture of the galaxy. Afterwards, the photograph slowly developed in space.

A vast and dark secret.

Guan felt a trace of sorrow in the face of such a majestic sight. He was thinking of the ecological sphere he had given to the Ring. It wasn't able to enjoy the gift for long. As it decayed into three-dimensional space, the Ring's internal four-dimensional structures were annihilated instantaneously. Those other dead or dying ships within the fragment would not be able to escape a similar fate in the end. In this vast universe, they could only persist for a time in the tiny four-dimensional corner.

Blue Space and Gravity sent out many probes to the dust belt. Other than scientific investigation, they also wanted to see if they could gather some useful resources. The Ring had decayed into common elements in three-dimensional space: mostly hydrogen and helium. These could be collected as fuel for nuclear fusion. However, since these elements existed mostly in the form of gas in the dust belt, they dissipated quickly, and in the end very little was collected. There were also some heavy elements, however, and they were able to gather some useful metals.

An independent hibernation ark would be constructed and powered by one of the seven fusion reactors on the two ships. Anyone wishing to go home would board this ark and return to the Solar System after a voyage of thirty-five years. The two ships would inform the Earth of the hibernation ark's course via a neutrino transmission so that the Earth could dispatch ships to meet it upon arrival. In order to prevent Trisolaris from locating Blue Space and Gravity based on this transmission, the neutrino transmission would only be made after the hibernation ark had already been on its way for some time. If the Earth could send out ships to assist the ark with deceleration before its arrival, more of the ark's fuel could be spent for acceleration and shorten the voyage to between ten and twenty years.

The two ships now had to consider their own future. A temporary council formed of the crews of both Blue Space and Gravity announced that everyone could pick between continuing the journey with the two ships or returning to the Solar System.

A month later, the hibernation ark and the two ships both embarked on their new voyages. The hibernation ark headed for the Solar System, while Blue Space and Gravity planned to divert around the four-dimensional fragment and then aim for a new target star system.

Assuming the Earth and the Solar System still existed by then.

Only about two hundred people chose to return. The rest didn't want to go back to that world doomed to destruction; they would rather stay with Blue Space and Gravity and continue into the unknown depths of space.

The luminous glow from the fusion reactors lit up the already sparse dust belt and cast it in a golden-red hue, like a warm sunset on Earth. Everyone, whether homeward bound or heading far away, felt hot tears filling their eyes. The beautiful space sunset quickly faded, and the eternal night fell over everything.

The two seeds of human civilization continued to drift into the depths of the starry sea. Whatever fate held in store for them, at least they were starting again.