Ezio put down his pack and from it produced the five keys.

He weighed the first one in his hand. “The end of the road,” he said, as much to himself as to Sofia.

“Not quite,” said Sofia. “First, we have to discover how to open the door.”

Ezio studied the keys and the slots into which they must fit. Symbols surrounding the slots gave him his first clue.

“They must—somehow—match the symbols on the keys,” he said, thoughtfully. “I know that Altaïr would have taken every precaution to safeguard this archive—there must be a sequence. If I fail to get that right, I fear the door may remain locked forever.”

“What do you hope to find behind it?” Sofia sounded breathless, almost—awed.

Ezio’s own voice had sunk to a whisper, though there was no one but her to hear him. “Knowledge, above all else. Altaïr was a profound man and a prolific writer. He built this place as a repository for all his wisdom.” He looked at her. “I know that he saw many things in his life and learned many secrets, both troubling and deep. He acquired such knowledge as would drive lesser men to despair.”

“Then is it wise to tap into it?”

“I am worried, it is true. But then”—he cracked a smile—“I am not, as you should know by now, a lesser man.”

“Ezio—always the joker.” Sofia smiled back, relieved that the tension had been broken.

He placed the torch he held in a sconce, where it gave them both enough illumination to read by. But he noticed that the symbols on the door had begun to glow with an indefinable light, scarcely perceptible, but clear, and that the keys themselves glowed, seemingly in response. “Have a careful look at the symbols on these keys with me. Try to describe them out loud as I look at the symbols on the door.”

She put on her glasses and took the first of the keys he gave her. As she spoke, he studied the markings on the door closely.

Then he gave a gasp of recognition. “Of course. Altaïr spent much time in the East, and gained much wisdom there.” He paused. “The Chaldeans!”

“You mean—this might have something to do with the stars?”

“Yes—the constellations. Altaïr traveled in Mesopotamia, where the Chaldeans lived—”

“Yes, but they lived two thousand years ago. We have books—Herodotus, Diodoros Siculus—that tell us they were great astronomers, but no detailed knowledge of their work.”

“Altaïr had—and he has passed it on here, encoded. We must apply our weak knowledge of the stars to theirs.”

“That is impossible! We all know that they managed to calculate the length of a solar year to within four minutes, and that’s pretty accurate, but how they did it is another matter.”

“They cared about the constellations and the movement of the heavenly bodies through the sky. They thought, by them, they could predict the future. They built great observatories—”

“That is pure hearsay!”

“It’s all we have to go on, and look—look here. Don’t you recognize that?”

She looked at a symbol engraved on one of the keys.

“He’s made it deliberately obscure—but isn’t that”—Ezio pointed—“the constellation of Leo?”

She peered at what he had shown her. “I believe it is!” she said, looking up, excited.

“And here”—Ezio turned to the door and looked at the markings near the slot he had just been examining—“here, if I am not mistaken, is a diagram of the constellation of Cancer.”

“But that is the constellation next to Leo, isn’t it? And isn’t it also the sign which precedes Leo in the Zodiac?”

“Which was invented by—”

“The Chaldeans!”

“Let’s see if this theory holds water,” said Ezio, looking at the next slot. “Here is Aquarius.”

“How apt,” Sofia joked, but she looked seriously at the keys. At last she held one up. “Aquarius is flanked by Pisces and Capricorn,” she said. “But the one that comes after Aquarius is Pisces. And here—I think—it is!”

“Let’s see if the others work out in a similar way.”

They worked busily and found, after only a matter of ten minutes more, that their supposition seemed to work. Each key bore the symbol of a constellation corresponding to a sign of the Zodiac, and each key sign corresponded to a slot identified with a constellation immediately preceding it in the Zodiac cycle.

“Quite a man, your Altaïr,” said Sofia.

“We’re not there yet,” Ezio replied. But, carefully, he put the first key into what he hoped was its corresponding slot—and it fit.

As did the other four.

And then—it was almost an anticlimax—slowly, smoothly, and soundlessly, the green door slid down into the stone floor.

Ezio stood in the entrance. A long hallway yawned before him, and, as he looked, two torches within, simultaneously and spontaneously, flared into life.

He took one from its sconce and stepped forward. Then he hesitated, and turned back to Sofia.

“You had better come back out of there alive,” she said.

Ezio gave her a mischievous smile and squeezed her hand tightly. “I plan to,” he said.

He made his way forward.

As he did so, the door to the vault slid closed again, so fast that Sofia hardly had time to react.

SEVENTY-FIVE

Ezio walked slowly down the hallway, which sloped ever downward and broadened out as he progressed. He scarcely had need of his torch since the walls were lined with them, and they flared alight, by some mysterious process, as he passed them. But he had no sense of unease, or trepidation. In a curious way, he felt as if he were coming home. As if something was nearing its completion.

At length, the hallway debouched into a vast, round chamber, 150 feet across and 150 feet high to the top of its dome, like the circular nave of some wondrous basilica. In the body of the room there were cases that must once have contained artifacts; but they were empty. The multiple galleries that ran round it were lined with bookshelf upon bookshelf—every inch of every wall was covered with them.

Ezio noticed, to his astonishment, that every single one of them was empty.

But he had no time to ponder the phenomenon, as his eye was drawn irresistibly to a huge oak desk on a high podium at the far end of the room, opposite the entrance. It was brightly lit from somewhere far above, and the light fell squarely on the tall figure seated at the desk.