They came at some length to a chamber, sealed by a massive ironbound door, which William unlocked with a key big enough to double as a bludgeon should the Bank come under attack. It was large for a subterranean vault, small for a parish church. Like a church, it had an aisle up the center, which had been made useful as a gutter. A trickle of groundwater two fingers wide meandered through crevices between close-set paving-stones. To either side of the aisle was a raised platform, also paved. Crates, lock-boxes, and money-bags had been piled up on these. William Ham led them up to the far end of the room, where the platforms gave way to a clear open space, and the trickle of aisle-water disappeared into a hole in the floor. He identified an open space at the end of a platform, and by a gesture indicated that Saturn should set the chest down there. Saturn did; but Solomon did not witness it, as he was inspecting the room.

Solomon shoved a sack of money aside to make a clear space on the platform, into which he spat, and then rubbed the saliva with a thumb until he had smeared away the patina of dust and congealed slime to reveal, in the light of Daniel’s lanthorn, a few chips of colored stone set into the surface. A glimpse of a Mosaic.

“Roman?” Daniel guessed. Solomon nodded.

“As I think I have now demonstrated, the plates will be perfectly safe in this sealed vault until the time has come to ship them to St. Petersburg,” William said to Solomon, as the ancient Jew came up to join them at the head of the chamber. But Solomon was staring at the floor. “Pray lend me your key,” he said, holding out a hand.

William Ham did not like this proposition at all. But he could come up with no reason to refuse. He placed it in Solomon’s hand. Solomon squatted down, felt the floor with his fingertips for some moments, then inserted the key’s handle-which looked a bit like a very ornate trowel-into the drain-hole. A bit of exploratory wiggling and prying led to the sudden appearance of a large crescent-moon-shaped crevice. Saturn stepped forward. “Careful!” Solomon continued, “it will be a well.”

“How do you know?”

“This is a Temple of Mithras, constructed by Roman soldiers,” Solomon said, “and every such temple contained a well.”

Saturn got his fingers into the crevice and pulled. A disk perhaps two feet in diameter came up out of the floor. It had been fashioned recently out of heavy planks. A lanthorn, let down into the cavity, revealed a well-shaft, lined with stones all the way down to the level of the water, which was perhaps three fathoms below.

“Your workmen found the well, and covered it,” said Solomon. “But more for their own safety, than the Bank’s security. I would wager the contents of this Bank that I could now leave the premises and meet you out in the street in half an hour’s time without passing out through the building’s front door.”

“That is a bet I could not accept, even if the money were mine to wager,” said William Ham, “for I can smell and feel the current of air rising up from the shaft as well as you.”

“Indeed, the well has a side-channel!” exclaimed Saturn, who was on his stomach with head and shoulders thrust down into the shaft. “About halfway down. I have a mind to fetch one of the workmen’s ladders and investigate!”

This was daft. But once Saturn had proposed, none could resist, it. Ladders were all over the place. They stabbed one down the well and planted it on the floor of the side-channel that Saturn had noticed. He went down first, and reported that the masons of the Temple of Mithras had carried out their duties well.

“As how could they not,” Solomon returned, “for Mithras was the god of contracts.”

“The god of contracts!?” exclaimed William Ham.

“Indeed,” said Solomon, “and so it is a good thing for you that you have founded your Bank on his Temple.”

“This Mithras does not appear in any Pantheon I have ever heard of.”

“He was not a god of Olympus but one that the Greeks borrowed from the Persians, who had in their turn borrowed him from Hindoostan. From the Greeks his cult spread to the Romans, and became popular around the hundredth year of what you call Anno Domini. Or, as I would put it, some years after the destruction of the Temple of Solomon. Especially among soldiers, such as garrisoned Londinium, along the banks of the Walbrook.”

Solomon had been clambering onto the ladder as he spoke.

“You aren’t going down there?”

“Mr. Ham, I was sent here by the Tsar to investigate the Bank’s security,” said Solomon, “and inspect it I shall!”

Daniel followed Solomon down the ladder. Three of them now squatted together in a vaulted tunnel that ran off into the earth, sloping gently down toward the well so that it, too, acted as a drain. William Ham was left to sit sentry in the Temple of Mithras, and to run for help if they never emerged. But after a very brief shuffle down the tunnel they sensed space above their heads, and found stone steps, which turned to the right and led them down to the level of the groundwater. A creek, perhaps eight feet in breadth, ran sluggishly off into the dark, wending round pilings, moles, and foundations one could only assume supported buildings up on the street. In rainy weather they might have had to stop and turn back. But it was the first day of August and the level did not rise above their ankles as long as they stayed along the side of the channel. So they ventured downstream, shining their lights on walls and foundations as they went, and speculating as to which belonged to which building.

“During the Plague,” Daniel said, “my uncle Thomas Ham-William’s father-enlarged the cellar of his goldsmith’s shop, which cannot be more than a stone’s throw from us. He discovered a Roman mosaic, and diverse pagan coins and artifacts. My wife in Boston is wearing one of them in her hair.”

“What did the mosaic depict?” Solomon asked.

“Some figures that called to mind Mercury. Mr. Ham styled it a Temple of Mercury and made of it a good omen. But it contained other images that would call his opinion into question-”

“Ravens?”

“Yes! How did you know?”

“Carox, the raven, was, to Persians, a messenger of the Gods-”

“As Mercury was to the Romans.”

“Indeed. The worshippers of Mithras believed that as the soul descended from the sphere of the fixed stars to be incarnated on Earth, it passed through all of the planetary spheres along the way, and was influenced by each in turn. In passing through the sphere of Venus the soul became amorous, and so on. The innermost sphere, and the last to wreak its influence on the soul, was that of Mercury or Corax. The practitioners of this cult believed that as the soul prepared for death, and a return to the sphere of the fixed stars, it must reverse that transmigration, shedding first the trappings of Mercury-Corax, then those of Venus, et cetera, and finally-”

“Saturn?” guessed Saturn.

“Indeed.”

“I am honored to be closest to the fixed stars, and least worldly of vices.”

“Accordingly, there were seven ranks. For each rank was a chamber-always subterranean. Your uncle’s cellar was that of Mercury-Corax, where new initiates were taken in. Later they would move through a gate or passage to the next chamber, which would have been decorated with images of Venus, and so on.”

“What was the big chamber under the Bank?”

“You shall be pleased to know it was the chamber of Saturn, for the highest-ranking members,” Solomon said.

“I did feel wondrously at home in the place!”

“If it is true that we are passing the foundations of the Ham goldsmith shop,” said Solomon, “then we are traversing the hierarchy in reverse order, following the same course as souls coming down from the C?lestial Sphere to be incarnated in the World.”

“Funny that,” Daniel said, “for I have just recognized the name of an old friend of mine, who’d be pleased to know where his work stood in the hierarchy.”