Langdon picked up the pace. "A secret location. Galileo's Illuminati needed to protect themselves from the Vatican, so they founded an ultrasecret Illuminati meeting place here in Rome. They called it The Church of Illumination."

"A little book written by a guy named Galileo."

Langdon strode powerfully up the deserted Via della Fondamenta toward the archives, his mind barely able to accept that he was about to be granted access. Vittoria was at his side, keeping pace effortlessly. Her almond-scented hair tossed lightly in the breeze, and Langdon breathed it in. He felt his thoughts straying and reeled himself back.

The Secret Vatican Archives are located at the far end of the Borgia Courtyard directly up a hill from the Gate of Santa Ana. They contain over 20,000 volumes and are rumored to hold such treasures as Leonardo da Vinci's missing diaries and even unpublished books of the Holy Bible.

She sounded surprised. "You don't mess around. What's in it?"

Vittoria said, "You going to tell me what we're looking for?"

"Sign towhat?"

"It is supposed to contain something calledil segno."

"Sign, clue, signal… depends on your translation."

"The sign?"

"Absolutely. In fact, they never revealed the location of their hideaway to anyone outside the brotherhood. This secrecy protected them, but it also posed a problem when it came to recruiting new members."

"Sounds like the Illuminati know how to keep a secret."

Langdon shook his head. "Galileo's Illuminati were not the least bit satanic. They were scientists who revered enlightenment. Their meeting place was simply where they could safely congregate and discuss topics forbidden by the Vatican. Although we know the secret lair existed, to this day nobody has ever located it."

"Exactly. Word of Galileo's brotherhood started to spread in the 1630s, and scientists from around the world made secret pilgrimages to Rome hoping to join the Illuminati… eager for a chance to look through Galileo's telescope and hear the master's ideas. Unfortunately, though, because of the Illuminati's secrecy, scientists arriving in Rome never knew where to go for the meetings or to whom they could safely speak. The Illuminati wanted new blood, but they could not afford to risk their secrecy by making their whereabouts known."

"Pretty bold calling a satanic lair achurch."

"They couldn't grow if they couldn't advertise," Vittoria said, her legs and mind keeping perfect pace.

Vittoria frowned. "Sounds like asituazione senza soluzione."

"Exactly. A catch-22, as we would say."

"So what did they do?"

"They were scientists. They examined the problem and found a solution. A brilliant one, actually. The Illuminati created a kind of ingeniousmap directing scientists to their sanctuary."

Langdon chuckled. "In a manner of speaking, it is. The Illuminati called their string of markers "The Path of Illumination," and anyone who wanted to join the brotherhood had to follow it all the way to the end. A kind of test."

Vittoria looked suddenly skeptical and slowed. "A map? Sounds careless. If a copy fell into the wrong hands…"

"It couldn't," Langdon said. "No copies existed anywhere. It was not the kind of map that fit on paper. It was enormous. A blazed trail of sorts across the city."

Vittoria slowed even further. "Arrows painted on sidewalks?"

Vittoria eyed him askance. "Sounds like a treasure hunt."

"In a sense, yes, but much more subtle. The map consisted of a series of carefully concealed symbolic markers placed in public locations around the city. One marker led to the next… and the next… a trail… eventually leading to the Illuminati lair."

"Sure," Langdon said, "if they hadknown about the markers. But they didn't. And they never noticed them because the Illuminati designed them in such a way that clerics would never suspect what they were. They used a method known in symbology asdissimulation."

"No. The path was hidden. A puzzle, constructed in such a way that only certain people would have the ability to track the markers and figure out where the Illuminati church was hidden. The Illuminati intended it as a kind of initiation, functioning not only as a security measure but also as a screening process to ensure that only the brightest scientists arrived at their door."

"I don't buy it. In the 1600s the clergy were some of the most educated men in the world. If these markers were in public locations, certainly there existed members of the Vatican who could have figured it out."

"But if the Vatican wanted to find the Illuminati," Vittoria argued, "couldn"tthey simply follow the markers?"

Langdon was impressed. "You know the term."

"Camouflage."

"Okay," Langdon said. "The Illuminati used the same concept. They created markers that faded into the backdrop of ancient Rome. They couldn't use ambigrams or scientific symbology because it would be far too conspicuous, so they called on an Illuminati artist -- the same anonymous prodigy who had created their ambigrammatic symbol 'Illuminati' -- and they commissioned him to carve four sculptures."

"Dissimulacione,"she said. "Nature's best defense. Try spotting a trumpet fish floating vertically in seagrass."

"Illuminatisculptures?"

"Yes, sculptures with two strict guidelines. First, the sculptures had to look like the rest of the artwork in Rome… artwork that the Vatican wouldnever suspect belonged to the Illuminati."

Langdon nodded, feeling a tinge of excitement, talking faster now. "And thesecond guideline was that the four sculptures had to have very specific themes. Each piece needed to be a subtle tribute to one of the four elements of science."

"Religiousart."

"Fourelements?" Vittoria said. "There are over a hundred."

Vittoria seemed mystified. "So this Illuminati artist created four pieces of art thatlooked religious, but were actually tributes to Earth, Air, Fire, and Water?"

The early cross, Langdon knew, was the most common symbol of the four elements -- four arms representing Earth, Air, Fire, and Water. Beyond that, though, there existed literallydozens of symbolic occurrences of Earth, Air, Fire, and Water throughout history -- the Pythagorean cycles of life, the ChineseHong -- Fan, the Jungian male and female rudiments, the quadrants of the Zodiac, even the Muslims revered the four ancient elements… although in Islam they were known as "squares, clouds, lightning, and waves." For Langdon, though, it was a more modern usage that always gave him chills -- the Mason's four mystic grades of Absolute Initiation: Earth, Air, Fire, and Water.

"Not in the 1600s," Langdon reminded her. "Early alchemists believed the entire universe was made up of only four substances: Earth, Air, Fire, and Water."

"Exactly," Langdon said, quickly turning up Via Sentinel toward the archives. "The pieces blended into the sea of religious artwork all over Rome. By donating the artwork anonymously to specific churches and then using their political influence, the brotherhood facilitated placement of these four pieces in carefully chosen churches in Rome. Each piece of course was a marker… subtly pointing to the next church… where the next marker awaited. It functioned as a trail of clues disguised as religious art. If an Illuminati candidate could find the first church and the marker for Earth, he could follow it to Air… and then to Fire… and then to Water… and finally to the Church of Illumination."

Vittoria was looking less and less clear. "And this has something to do with catching the Illuminati assassin?"

Langdon smiled as he played his ace. "Oh, yes. The Illuminati called these four churches by a very special name. The Altars of Science."

Langdon gave her a smile. "Four cardinals. Four churches. The four altars of science."

Vittoria frowned. "I'm sorry, that means noth --" She stopped short."L"altare di scienza?" she exclaimed. "The Illuminati assassin. He warned that the cardinals would be virgin sacrifices on the altars of science!"

"I believe so, yes."

"Why not?" Langdon replied. "Very few historians know about these sculptures. Even fewer believe they exist. And their locations have remained secret for four hundred years. No doubt the Illuminati trusted the secret for another five hours. Besides, the Illuminati don"tneed their Path of Illumination anymore. Their secret lair is probably long gone anyway. They live in the modern world. They meet in bank boardrooms, eating clubs, private golf courses. Tonight theywant to make their secrets public. This is their moment. Their grand unveiling."

"But why would the killer have given us that clue?"

She looked stunned. "You're saying the four churches where the cardinals will be sacrificed are thesame four churches that mark the ancient Path of Illumination?"

Langdon feared the Illuminati unveiling would have a special symmetry to it that he had not yet mentioned. The four brands. The killer had sworn each cardinal would be branded with a different symbol. Proof the ancient legends are true, the killer had said. The legend of the four ambigrammatic brands was as old as the Illuminati itself: earth, air, fire, water -- four words crafted in perfect symmetry. Just like the word Illuminati. Each cardinal was to be branded with one of the ancient elements of science. The rumor that the four brands were inEnglish rather than Italian remained a point of debate among historians. English seemed a random deviation from their natural tongue… and the Illuminati did nothing randomly.

Langdon turned up the brick pathway before the archive building. Ghastly images thrashed in his mind. The overall Illuminati plot was starting to reveal its patient grandeur. The brotherhood had vowed to stay silent as long as it took, amassing enough influence and power that they could resurface without fear, make their stand, fight their cause in broad daylight. The Illuminati were no longer about hiding. They were about flaunting their power, confirming the conspiratorial myths as fact. Tonight was a global publicity stunt.

When the guard saw them, he stopped in his tracks. He stared at them, as though he thought he was hallucinating. Without a word he turned away and pulled out his walkie-talkie. Apparently incredulous at what he was being asked to do, the guard spoke urgently to the person on the other end. The angry bark coming back was indecipherable to Langdon, but its message was clear. The guard slumped, put away the walkie-talkie, and turned to them with a look of discontent.

Vittoria said, "Here comes our escort." Langdon looked up to see a Swiss Guard hurrying across an adjacent lawn toward the front door.

Not a word was spoken as the guard guided them into the building. They passed through four steel doors, two passkey entries, down a long stairwell, and into a foyer with two combination keypads. Passing through a high-tech series of electronic gates, they arrived at the end of a long hallway outside a set of wide oak double doors. The guard stopped, looked them over again and, mumbling under his breath, walked to a metal box on the wall. He unlocked it, reached inside, and pressed a code. The doors before them buzzed, and the deadbolt fell open.

The guard turned, speaking to them for the first time. "The archives are beyond that door. I have been instructed to escort you this far and return for briefing on another matter."

"You're leaving?" Vittoria demanded.

"But how do we getout?"

Vittoria made some comment, but Langdon did not hear. His mind was fixed on the double doors before him, wondering what mysteries lay beyond.

"Monodirectional security. You will have no difficulties." That being the entirety of the conversation, the guard spun on his heel and marched off down the hall.

"Swiss Guards are not cleared for access to the Secret Archives. You are here only because my commander received a direct order from the camerlegno."