"Pluto is at its apogee, rather far from the two-dimensional space. The Federation Government is about to issue a formal attack alert to the world, and many ships will be headed there. Although the final result will be the same, at least there will be more time left."

"The Federation Government would like to ask you to do something."

"Let's head back to the Earth," Cheng Xin said softly. This was the first idea that floated up through the chaos and darkness of her jumbled thoughts.

"The Earth is not a bad place to wait for the end. A falling leaf seeks to return to the root. But we hope Halo will go to Pluto," Cao Bin said.

"That's not long enough to be worth worrying about. Let's go back to Earth," said AA.

"Pluto?"

"The entire Solar System within the Kuiper Belt will collapse into two dimensions in eight to ten days."

"How much longer?"

"What can we possibly do now?"

"Not anything important. There's nothing important now. But someone came up with the idea that theoretically, there might exist image-processing software that could process a two-dimensionalized image of a three-dimensional object and re-create the three-dimensional object. We hope that in the distant future, some intelligent civilization might re-create a three-dimensional representation of our world from its two-dimensionalized image. Though it would be nothing more than a dead representation, at least human civilization would not be forgotten."

"Luo Ji? He's alive?!" AA cried out.

"Yes. He's almost two hundred."

"Also, Luo Ji is on Pluto. He wants to see you."

"The Earth Civilization Museum is on Pluto. A large portion of humanity's precious artifacts are stored there. The museum is buried under the surface, however, and we are concerned that during the process of falling into the plane, these artifacts would be mixed together with the strata of the crust and their structures would be damaged. We'd like to ask you to carry some of the artifacts away from Pluto on Halo and scatter them in space so that they can fall into two dimensions separately. This way, their structures would be preserved without harm in two dimensions. I guess this counts as a kind of rescue mission… Of course, I admit that the idea is nearly science fiction, but doing something now is better than doing nothing."

"All right. Let's go to Pluto," Cheng Xin said. In the past, this would have been an extraordinary journey. But now, nothing mattered.

A pleasant male voice spoke up. "Do you wish to go to Pluto?"

"Yes. What do we do?"

"You just have to confirm the request. There's no need to do anything. I will complete the voyage for you."

"Yes, we want to go to Pluto."

"I'm Halo, or Halo's AI. Do you wish to go to Pluto?"

"Authorization confirmed. Processing. Halo will accelerate at 1G in three minutes. Please pay attention to the direction of gravity."

"Who are you?" asked AA.

Outside the porthole, they could see a few blue reflections appearing on the shell of the combined city -- reflections of Halo's nozzle lights. Cheng Xin and AA fell to one side of the spherical hall and felt their bodies grow heavier. The acceleration soon reached 1G. After the two of them -- still weak from hibernation -- struggled up and looked outside the porthole again, they saw the entirety of Jupiter. It was still immense, and shrinking at too slow a rate to be perceived.

Cao Bin said, "Good. Better leave early. After the attack alert is issued, there might be total mayhem. Hopefully we'll get a chance to talk again." He closed the window link before AA and Cheng Xin could say good-bye. At this moment, AA, Cheng Xin, and Halo were not his top priorities.

The ship's AI led AA and Cheng Xin on a tour of the ship to familiarize them with it. Like its predecessor, this new Halo was still a small stellar yacht with a maximum capacity of four. Most of the space on the ship was taken up by the ecological cycling system. By conventional measures, the ecological cycling system was extremely redundant -- a volume of space that would have supported forty was used to provide for only four. The system was divided into four identical subsystems, linked together and acting as each other's backups. If any of the four failed accidentally, the other three could bring it back to life. Halo's other distinguishing characteristic was the ability to land directly on a medium-sized solid planet. This was a rare design choice among stellar ships -- similar ships typically used shuttles to carry landing parties onto planets. Directly descending into a planet's deep gravity well required the ship to have a very strong hull, which greatly increased the cost. Moreover, the need for atmospheric flight required a streamlined profile, which was also very rare among stellar ships. All of these design features meant that if Halo could find another Earthlike planet in outer space, it could act as a habitable base for the crew on the surface of the planet for a considerable amount of time. Maybe it was these characteristics of Halo that led to it being chosen for the artifact-rescuing mission to Pluto.

There were numerous other unusual features on the yacht. For instance, it had six small courtyards, each about twenty to thirty square meters in size. Each courtyard automatically adjusted to the direction of gravity under acceleration, and, during coasting, spun independently within the ship to generate artificial gravity. Each courtyard displayed a different natural scene: a green lawn with a babbling brook running through the grass; a small copse with a spring in the middle; a beach with waves of clear water throwing up surf… These scenes were small but exquisite, like a string of pearls made of the best parts of the Earth. On a small stellar spaceship, such a design was extremely luxurious.

Cheng Xin felt both distressed and sorry for Halo. Such a perfect little world was soon going to be turned into a slice without thickness. She tried to avoid thinking about those other grander things facing imminent destruction -- annihilation covered the sky of her thoughts like a giant pair of black wings, and she dared not look directly up at it.

The attack takes the form of a dimensional strike, which will collapse the space around the Solar System from three dimensions to two dimensions. The result will be the complete destruction of all life.

Five hours ago, the advance warning system confirmed that a dark forest strike has been initiated against our world.

Two hours after departure, Halo received the formal dark forest attack alert issued by the Solar System Federation Government. The president, a beautiful woman who looked very young, made the announcement. She stood in front of the blue flag of the Federation and spoke without expression. Cheng Xin noticed that the blue flag resembled the ancient UN flag, though a diagram of the Sun replaced the diagram of the Earth. This most important document, marking the end of human history, was very short:

We have confirmed that the escape velocity for the collapsing region is the speed of light.

The process is estimated to take eight to ten days. At this moment, the collapse is ongoing and the rate and extent of collapse are rapidly growing.

An hour ago, the Federation Government and Parliament have passed a new resolution that repeals all laws regarding Escapism. However, the government wishes to remind all citizens that the escape velocity far exceeds the maximum velocity of all human space vehicles. The probability of a successful escape is zero.

The Federation Government, Parliament, the Supreme Court, and Federation Fleet will carry out their duties until the end.

AA and Cheng Xin didn't bother to watch more news. It was possible that, just like Cao Bin said, the Bunker World had approached paradise. They wanted to see what paradise looked like, but they didn't dare look. If everything was heading toward ruination, the more beautiful it all was, the more pain they would suffer. In any event, it was a paradise that was collapsing in the terror of death.

Halo stopped accelerating. Behind it, Jupiter became a small yellow dot. The next few days of the voyage were spent in the uninterrupted slumber produced by the sleep-aid machine. In this lonely voyage through the night before the end, just the unstoppable mad imaginings were enough to make anyone fall apart.

Halo continued to descend. On Earth, this would have involved a soul-stirring atmospheric reentry, but Halo continued to fly through the silent vacuum, decelerating by the power of its own thrusters. On the blue-black ground below, an attention-grabbing line of white text appeared: EARTH CIVILIZATION

Halo's AI awakened AA and Cheng Xin from their dreamless sleep as the ship reached Pluto.

Out of the porthole and on the monitor they could see the entirety of Pluto. Their initial impression of the dwarf planet was one of darkness, like an eye that remained perpetually shut. This far from the Sun, the light was extremely dim. Only when Halo entered low orbit could they see the colors on the surface of the planet: Pluto's crust appeared to be made of patches of blue and black. The black was rocks -- not necessarily black in color, but the light was too dim to tell otherwise. The blue was solidified nitrogen and methane. Two centuries ago, when Pluto had been near its perigee and inside Neptune's orbit, the surface would have looked completely different. The ice cover would have partially melted and produced a thin atmosphere. From the distance, it would have appeared a deep yellow.

The text was written in the modern script that mixed Latin and Chinese elements. After it, there were a few more lines of smaller text repeating the same thing in different scripts. Cheng Xin noticed that none of them said "museum." The yacht was still about one hundred kilometers above the surface, which meant that the text was gigantic. Cheng Xin couldn't make an exact estimate of the size of the characters, but she was certain that these were the largest written characters ever produced by humankind, each big enough to contain a city. By the time Halo was only about ten thousand meters above the surface, one of the large characters took up the entire field of view. Finally, Halo touched down on the broad landing field, which was the topmost dot in the Chinese character qiu (球), a part of the word Earth.

With the guidance of the ship's AI, Cheng Xin and AA put on light space suits and exited Halo onto the surface of Pluto. Given the frigid surroundings, the heating systems in their space suits were operating at maximum power. The landing field was empty, white, and seemed to phosphoresce in the starlight. The numerous burn marks left on the ground indicated that many spacecraft had once landed and taken off here, but Halo was the only ship here now.

During the Bunker Era, Pluto was akin to Antarctica on ancient Earth. No one lived here permanently, and few came to visit.

Halo turned on its searchlights. Due to the lack of atmosphere, there wasn't a visible beam of light. It cast a circle of light on a distant rectangular object. This black monolith was the only protrusion above the white ground. It gave off an eerie sense of simplicity, as though it was an abstraction of the real world.

"I don't know what it is, but I don't have a good feeling about it."

In the sky, a black sphere moved rapidly among the stars. It was large, but the surface was shrouded in darkness: Charon, Pluto's moon. Its mass was a tenth of Pluto's, and the two almost formed a double-planet system, revolving around a common center of mass.

Cheng Xin and AA headed for the monolith. Pluto's gravity was only one-tenth of the Earth's, and so they proceeded by leaping. Along the way, they noticed a row of arrows pointing toward the monolith on the ground. Only when they reached the monolith did its immensity impress itself on their minds. When they looked up, it was as though a chunk had been taken out of the starry sky. They looked around and saw that there were rows of arrows coming from other directions, all pointing toward the monolith. At the foot of the monolith was another prominent protuberance: a metal wheel about a meter in diameter. To their surprise, they found the wheel to be hand-operated. Above the wheel was a diagram formed from white lines against the black surface of the monolith. Two curved arrows indicated the directions in which the wheel could be turned. Next to one of the arrows was a drawing of a half-open door, while the other bore a drawing of a shut door. Cheng Xin turned to survey the arrows on the ground pointing to the monolith. All the simple, clear, wordless instructions gave her a strange feeling, which AA voiced.

"That looks a bit familiar," Cheng Xin said.

They turned the wheel clockwise. The wheel was stiff, but eventually a door opened in the surface of the monolith. Some gas escaped, and the water vapor within quickly deposited into ice crystals that glinted in the searchlight. They entered the door and saw another door facing them, also operated by a wheel. This time, there were simple written instructions above the wheel, informing them that they were in an air lock and needed to close the first door before opening the second. This was unusual, since as early as the end of the Crisis Era, pressurized buildings could open their doors directly to vacuum without needing an air lock.

Cheng Xin and AA turned the wheel on the inside of the door they had entered to shut it. The searchlight was cut off. They were about to turn on the lights on their space suits to hold off the terror of darkness when they noticed a small lamp in the ceiling of the narrow air lock. This was the first sign they had seen of electricity. They began to turn the wheel to open the second door. Cheng Xin was certain that even if they hadn't closed the first door, they would still have been able to open the second. The only thing that prevented air from leaking was following the instructions. In this low-technology environment, there was no automatic mechanism to prevent errors.

"These things… I don't think they're intended for humans."

The rush of air almost toppled them, and the rapidly warming temperature fogged their visors. But the space suits told them that the external air pressure and composition was breathable; they could open their helmets.

"There's no elevator?" AA asked. She was frightened by the steep way down.

"An elevator might break down over time. This building was intended to last through geologic eons." The voice came from the other end of the tunnel, where an old man appeared. In the dim light, his long white hair and beard floated in the low gravity. They seemed to be giving off their own light.

They saw a tunnel lit by a series of dim lamps heading into the earth. The dark walls of the tunnel swallowed up the dim light they emitted so that, between the cones of light, all was darkness. The floor of the tunnel was a smooth incline. Although the angle was steep, close to forty-five degrees, there were no stairs. This design was probably motivated by two considerations: There was no need for stairs in low gravity, or the path wasn't meant for humans.

"Who else? Children, my legs don't work so well anymore, so forgive me for not coming up to meet you. Come on down by yourselves."

"Are you Luo Ji?" AA shouted.

Cheng Xin and AA descended the incline in leaps. Due to the low gravity, this wasn't a very dangerous maneuver. As they approached the old man, they saw that he was indeed Luo Ji. He wore a long white changshan, a Chinese-style robe, and leaned against a cane. His back was slightly bowed, but his voice was hale and loud.

"Haha, there's no need for that." Luo Ji waved his hands. "We used to be… colleagues." He looked at Cheng Xin and in his eyes was a surprised delight that almost seemed incongruent with his age. "You're still so young. There was a time when I saw you only as the Swordholder, and then, gradually, you became a lovely young woman. Haha…"

At the bottom of the incline, Cheng Xin bowed deeply. "Honored Elder, hello."

In Cheng Xin's and AA's eyes, Luo Ji had also changed. The stately Swordholder was gone. They didn't know that the cynical, playful Luo Ji in front of them now was a return to the Luo Ji from four centuries ago, before he had become a Wallfacer. That Luo Ji had returned, as if awakening from hibernation, but the passage of time had moderated him, and filled him with more transcendence.

"You and I have both busied ourselves for nothing," said Luo Ji to Cheng Xin.

"Of course, child." He pointed behind him with his cane. "Those idiots all left on spaceships. They knew that they ultimately couldn't escape, but they still tried to run. Foolish."

"Do you know what has happened?" AA asked.

He meant the other workers in the Earth Civilization Museum.

It took Cheng Xin a bit of time before she understood what he meant, but the flood of emotions and memories was interrupted by Luo Ji's next words. "Forget it. Carpe diem has always been the right path. Of course there's not much diem now for carpe, but we need not look for trouble. Let's go. You don't need to help support me. You haven't even learned how to walk properly around here yet."

After a while, space opened up before them. Cheng Xin and AA realized that they were now in a much wider and bigger tunnel -- a cavern, really. The ceiling was high above, but the space was still only lit by a dim row of lights. The cavern looked very long, and the other end was not visible.

Given Luo Ji's advanced age of two hundred, the difficulty of locomotion under such low gravity wasn't moving too slow, but too fast. The cane wasn't so much a support as a decelerator.

"Where are the artifacts?"

"This is the main body of the museum," Luo Ji said.

"In the halls down at the other end. Those aren't so important. How long can they keep? Ten thousand years? A hundred thousand years? A million, at the most. Practically all of them will have turned to dust by then. But these --" Luo Ji pointed around them. "-- were intended to be preserved for hundreds of millions of years. Why, do you still think this is a museum? No, no one visits here. This is not a place for visitors. All of this is but a tombstone -- humankind's tombstone."

Cheng Xin looked around the empty, dim cavern and thought back to all she had seen. Indeed, everything was filled with hints of death.

"How did such an idea come up?" AA looked all around.

"You ask that because you're too young." Luo Ji pointed to Cheng Xin and himself. "During our time, people often planned for their own gravesites while they were still alive. Finding a graveyard for humanity isn't so easy, but erecting a tombstone is doable." He turned to Cheng Xin. "Do you remember Secretary General Say?"

"She was a true visionary. I've thought of her often through the years. Could she really have died almost four hundred years ago?" Luo Ji leaned on the cane with both hands and sighed. "She was the one who thought of this first. She wanted to do something so that humanity would leave behind a legacy that could be preserved for a long time after our civilization was gone. She planned an unmanned ship filled with cultural artifacts and information about us, but it was deemed a form of Escapism, and the project halted with her death. Three centuries later, after the Bunker Project began, people remembered it. That was a time when people worried that the world was going to end any moment. The new Federation Government decided to build a tombstone at the same time that the Bunker Project was built, but it was officially referred to as the Earth Civilization Museum so as not to be seen as a sign of pessimism. I was named the chair of the tombstone committee.

Four centuries ago, while she had worked for the PIA, Cheng Xin had met Say, the UN secretary general, a few times at various meetings. The last time was at a PIA briefing. Wade was there, too. On a big screen, Cheng Xin had given Say a PowerPoint presentation about the Staircase Project. Say had sat there quietly and listened to the whole thing without asking any questions. Afterwards, Say walked next to Cheng Xin, leaned in, and whispered, "We need more people to think like you."

Cheng Xin nodded. "Of course."

"At first, we engaged in a large research project to study how to preserve information across geologic eons. The initial benchmark was a billion years. Ha! A billion. Those idiots thought that would be easy -- after all, if we could build the Bunker World, how difficult could this be? But they soon realized that modern quantum storage devices, while capable of storing a whole library in a grain of rice, could only preserve the information without loss for about two thousand years. After that, decay would make it impossible to decode. As a matter of fact, that only applied to the highest-quality storage devices. Two-thirds of more common varieties failed within five hundred years. This suddenly transformed the project from a detached, contemplative matter into an interesting practical problem. Five hundred years was real -- you and I came from only four hundred years ago, right? So the government stopped all work on the museum and directed us to study how to back up important data about the modern world so that it could be read in five hundred years, heh heh… Eventually, a special institute had to be set up to tackle the problem so that the rest of us could focus on the museum, or tombstone.

"We informed the government that, given current technology, preserving ten gigabytes of images and one gigabyte of text -- that was the basic information requirement for the museum -- for one billion years was impossible. They wouldn't believe us, and we had to show them the evidence. Finally, they agreed to lower the requirement to one hundred million years.

"Scientists realized that in terms of data longevity, storage devices from our time were better. They found some USB flash drives and hard drives from the Common Era and some still had recoverable data! Experiments showed that if these devices were of high quality, information was safe on them for about five thousand years. The optical disks from our era were especially resilient. When made from special metal, they could reliably preserve data for a hundred thousand years. But none of these were a match for printed material. Special ink printed on composite paper could be read in two hundred thousand years. But that was the limit. Our conventional data storage techniques could preserve information for two hundred thousand years, but we needed to get to a billion!

"The research continued, but there was no progress. The other specialists had obviously reached conclusions, but they didn't want to speak up. I told them, 'Don't worry about it. Whatever conclusions you've reached, no matter how bizarre or outrageous, we must accept them if there are no alternatives.' I promised them that there was nothing that could be more bizarre and outrageous than what I'd gone through, and I would not laugh at them. So they told me that, according to the most advanced theories and techniques in every field, based on extensive theoretical research and experimentation, through analysis and comparison of multiple proposals, they did find a way to preserve information for about one hundred million years. And they emphasized that this was the only method known to be practicable. Which is --" Luo Ji lifted the cane over his head, and as his white hair and beard danced in the air, he resembled Moses parting the Red Sea. Solemnly, he intoned, "-- carving words into stone."

"But this was still an extremely difficult task. We looked for information that had survived for such a long time. Patterns drawn on prehistoric pottery survived about ten thousand years. Cave paintings in Europe were from about forty thousand years ago. If you count the markings made on stones back when our ancestors, the hominids, made the first tools as information, then the earliest instances occurred during the Pliocene, two point five million years ago. And we did indeed find information left one hundred million years ago, though it wasn't left by humans: dinosaur footprints.

AA giggled. But Cheng Xin wasn't laughing. She was stunned.

Cheng Xin walked to one of the walls. In the dim light, she saw that it was covered with dense, carved text, as well as images in relief. The wall was not the original rock, but seemed to have been infused with metal, or perhaps the surface had been coated with some durable titanium alloy or gold. Fundamentally, however, it was no different from carving words into stone. The carved text wasn't small: each character or letter was about a square centimeter. This was another feature intended to help with information longevity, as smaller text tended to be harder to preserve.

"These lamps are really strange," said AA.

Cheng Xin looked at the lamp on the cave wall. First, she noticed its shape: an arm poking out of the wall holding a torch. She thought this was a familiar design, but clearly that wasn't what AA meant. The torch-shaped lamp seemed very clumsy. The size and structure resembled an ancient searchlight, but the light it emitted was very weak, about the same as an ancient twenty-watt incandescent light bulb. After passing through the thick lampshade, the light was not much brighter than a candle.

"Carving words into stone." Luo Ji pointed at the walls of the cavern.

"Of course, this approach meant that the information storage capacity was greatly reduced, leaving us with less than one-ten-thousandth of the planned amount. But they had no choice but to accept this limitation," Luo Ji said.

Cheng Xin took off a space suit glove and caressed the characters carved into the cold stone. Then she leaned against the cave wall and stared woodenly at the lamps. She realized where she had seen this design: the Panthéon in Paris. A hand holding a torch just like the one on Rousseau's tomb. The faint yellow lights before her now didn't seem to be electric, but like tiny flames about to go out.

Luo Ji said, "Back that way is the machinery dedicated to providing electricity to this complex, like a power plant. This lamp is an amazing accomplishment. There's no filament or excitable gas inside, and I don't know what the luminous element is, but it can continue to glow for a hundred thousand years. The doors you came through should continue to be operable under normal conditions for five hundred thousand years. After that, the doors will deform and whoever wants to come in will have to break them down. By then, these lamps will have gone out more than four hundred thousand years earlier, and darkness will reign here. But that will be but the start of the journey of a hundred million years."

"Ah, I used to love to talk, and then I forgot how. But now I've learned again. I can't stop chattering, like a kid. I hope I'm not bothering you?"

Cheng Xin examined the carvings on the wall carefully. They began with the relief carving of a man and a woman, perhaps an attempt to show future discoverers what humans looked like. But unlike the stiff bearing of the drawings of the man and woman on the metal plaque carried by the Pioneer probes during the Common Era, the two cave carvings were done with lively expressions and postures, evoking Adam and Eve.

"You are not very talkative," Luo Ji said. His voice was suffused with a solicitousness that Cheng Xin had long missed.

"She's always been like that," said AA.

Carving words into stone.

Cheng Xin struggled to smile. "Not at all. It's just that… looking at all this, I don't know what to say."

True. What was there to say? Civilization was like a mad dash that lasted five thousand years. Progress begot more progress; countless miracles gave birth to more miracles; humankind seemed to possess the power of gods; but in the end, the real power was wielded by time. Leaving behind a mark was tougher than creating a world. At the end of civilization, all they could do was the same thing they had done in the distant past, when humanity was but a babe:

Under the Greek letters, Cheng Xin saw another relief, which seemed to portray ancient scholars in simple robes debating in an agora surrounded by stone columns.

Cheng Xin had a strange idea. She turned back and looked near the beginning of the cave carvings, but didn't find what she was looking for.

Cheng Xin walked along the wall. After the man and the woman came some hieroglyphs and cuneiforms, probably copied from ancient artifacts -- it was possible that some of them were not even intelligible to modern men and women, and if so, how would future extraterrestrial discoverers understand them? Going further, Cheng Xin saw Chinese poetry -- or, at least, she could tell the carvings were poetry based on the arrangement of the characters. But she didn't recognize any of the characters; she could only tell they were in Great Seal Script.

"That's the Classic of Poetry, from a millennium before the time of Christ," Luo Ji said. "If you keep on walking, you'll see fragments of Classical Greek philosophy. To see letters and characters that you can read, you'll have to walk tens of meters."

"Yes. Isn't there some system to help with interpretation?"

"You are looking for a Rosetta Stone?" Luo Ji asked.

AA looked at the cave wall and then stared at Luo Ji. "You're saying that we've carved things here that we don't even understand with the hope that someday, some extraterrestrial will be able to read them?"

"Child, we're talking about carving in stone, not a computer. How can we possibly fit something like that here?"

True, to the extraterrestrial discoverers of the far future, the human classics left on the walls here would probably resemble Linear A, Cretan hieroglyphics, and other ancient scripts that no one could read. Perhaps there was no realistic hope that anyone would. By the time the builders of this monument truly understood the power of time, they no longer believed that a vanished civilization could really leave behind any marks that would last through geologic eons. As Luo Ji had said, this wasn't a museum.

A museum was built for visitors; a tombstone was built for the builders.

"I often stroll around here thinking my own crazy thoughts." Luo Ji paused and pointed at a relief carving of an ancient soldier in armor and wielding a spear. "This is about the conquests of Alexander the Great. If he had kept on going a bit farther east, he would have encountered the Qin at the end of the Warring States Period -- what would have happened then? And how would history have changed?" They walked some more, and he pointed at the cave wall again. By now, the characters carved on the wall had turned from Small Seal Script to Clerical Script. "Ah, we've reached the Han Dynasty. From here to later, China completed two unifications. Are a unified territory and a unified system of thought good things for civilization as a whole? The Han Dynasty ended up endorsing Confucianism above all, but if the multiplicity of schools of thinking during the Spring and Autumn Period had continued, what would have happened later? How would the present be different?" He waved his cane around in a circle. "At every moment in history, you can find endless missed opportunities."

The three continued onward, and Luo Ji's cane tapped along the ground rhythmically.

"Interesting! I hope you're right… Look, this is where the artifacts are kept. We have a total of three halls."

"Like life," said Cheng Xin softly.

"Oh, no no no." Luo Ji shook his head vigorously. "At least not for me. I don't think I've missed anything, haha." He looked at Cheng Xin. "Child, do you think you've missed out? Then don't let opportunities go by again in the future."

"Who knows? Perhaps a flat two-dimensional civilization will be able to see all this," said AA.

They reached the end of the cave. Turning around to survey this underground tombstone, Luo Ji sighed. "We had designed this place to last a hundred million years, but it won't even survive a hundred."

Luo Ji tapped one of the nearby boxes with his cane. "As I said, these are not so important. Most of these objects have longevities shorter than fifty thousand years, though some of the statues can survive up to a million years. But I suggest you not move the statues: Though the gravity makes them easy to move, they take up too much space… All right, pick whatever you like."

"There's no future now," said AA coldly. She wondered if Luo Ji was suffering from dementia.

Cheng Xin and AA saw space open up before them once more. The room they were in didn't resemble an exhibit hall so much as a warehouse. All the artifacts were placed in identical metal boxes, and each box was labeled in detail.

Luo Ji picked up a set of work overalls from on top of another box and retrieved a small knife and screwdriver from the pockets. "The frame takes up a lot of space. You can take it off."

AA picked up the screwdriver, but before she could get started on the painting, Cheng Xin cried out. "No!" The painting was Van Gogh's Starry Night.

Cheng Xin's surprise wasn't just because the painting was valuable. She had seen it once before. Four centuries ago, right after she had started working at the PIA, she had visited New York's Museum of Modern Art on a weekend and saw a few of Van Gogh's paintings. Van Gogh's representation of space had left a deep impression on her. In his subconscious, space seemed to have structure. Cheng Xin wasn't an expert in theoretical physics back then, but she knew that according to string theory, space, like material objects, was made up of many microscopic vibrating strings. Van Gogh had painted these strings: In his paintings, space -- like mountains, wheat fields, houses, and trees -- was filled with minute vibrations. Starry Night had left an indelible mark in her mind, and she was amazed to see it again four centuries later on Pluto.

"I guess paintings take up a lot of space, too," said AA.

AA looked around excitedly. "I suggest we take paintings. We can forget about old classics and ancient manuscripts -- no one will understand those." She walked in front of one of the metal boxes and pushed what looked like a button on top, but the box didn't open by itself, and there were no instructions. Cheng Xin walked over and struggled to lift the cover open. AA took out an oil painting.

"Get rid of the frame. That way, you can take more." Luo Ji waved his cane carelessly. "Do you think these objects are still worth a city's ransom? Now, even a city is worthless."

And so they pried away the frame that was perhaps five centuries old, but they kept the hard backing to avoid damaging the painting by bending the canvas. They continued to do the same to other oil paintings, and soon, empty frames littered the floor. Luo Ji came over and put his hand on a small painting.

Cheng Xin and AA moved the painting aside and set it on top of a box next to the wall. They were surprised to see that it was the Mona Lisa.

Cheng Xin and AA continued to work at disassembling frames. AA whispered, "Clever old man. He kept the most expensive piece for himself."

"Maybe he once loved a girl named Mona Lisa?"

"Would you leave this one for me?"

Luo Ji sat next to the Mona Lisa and caressed the ancient frame with one hand. He muttered, "I didn't know you were here. Otherwise I could have come to see you often."

"I don't think that's the reason."

Inside the grand tomb under the surface of Pluto, lit by the dim lamps that could shine for a hundred thousand years, Mona Lisa's smile seemed to appear and disappear. The smile had puzzled humankind for nearly nine centuries, and it looked even more mysterious and eerie now, as though it meant everything and nothing, like the approaching Death.

Cheng Xin saw that he wasn't looking at the painting. His eyes stared ahead as if looking into the depths of time. Cheng Xin saw that his ancient eyes were filled with tears, and she wasn't sure if she was mistaken.