As we walked back through the dusky street toward the yard, Brunel said thoughtfully, "Where do they go?"

I understood him at once. "I do not send them anywhere. I simply give them rest."

"You have no misgivings about what you do? Some would say that it is very like murder."

"They are not souls, Mr. Brunel; if they were, it would not be my place to send them on. What people commonly call ghosts are not souls with some unfinished business that delays their passing over, but shells cast off at moments of intense, concentrated emotion. It is true that many are cast off at death, but not all who die cast off a ghost, and not all ghosts are cast off by the dying. Most are not long-lived, and almost all are damaged or deformed representations of the people who produce them. There are very few with whom you could hold a sensible conversation, and even fewer which would not feel relief at the moment of dissolution. They are poor frightened creatures that cling to a familiar place or a familiar person. Most often they haunt the person or the body of the person who cast them off, and those last often become attached to whoever finds that body. I believe that I am able to make a guess at Mr. Jake Mullins's occupation, even without your mention of 'inquest money.' "

"He is a river finder. He and his kind dredge for all kinds of things by day—coal, animal bones, pieces of metal. They ferry contraband from one place to another, too, usually items of cargo from ships waiting to be unloaded."

"But at night, Mr. Mullins and his fellows look for bodies of the drowned."

"He gets the reward, if there is one, and in any case five shillings from the police. Was he much … inhabited?"

"I should say that he has been pretty successful in his searches."

"I should have warned you. I did not think."

"None of them were harmful. They were mostly pathetic scraps. It was the number that astonished me."

Brunel laughed. "A few days ago, I would have thought myself mad if I had found myself in the middle of a conversation like this."

"A few days ago, you would not have considered attending a seance."

"Perhaps I am mad," Brunel said thoughtfully. "The dreams were certainly bad enough to be the dreams of a mad man. I suppose it would be too simple to think that I am haunted by the ghost of Ulpius Silvanus?"

"There was no ghost that I could detect. Of course, there are lesser creatures than ghosts. Imps of delirium and madness, and the like …"

Brunel looked at me from beneath the brim of his stovepipe hat. "What is wrong, Carlyle?"

"I have been a fool. There are creatures lesser than ghosts, and there are greater creatures, too. Something has been awakened, I think. Something very old, and once possessed of great power. Think, Mr. Brunel. What kind of stone commonly becomes the focus of human desires?"

Brunel had the quickest mind of any man I have ever met. After only a moment, he said, "You think that Dr. Pretorius is searching for an altar."

"I do indeed. And because the remains we found are almost certainly that of a Roman soldier, I believe that it is an altar that was dedicated to some pagan god long before Christianity enlightened these shores. Pretorius spoke of stones under the city. And is the city not built, layer upon layer, upon its own past, like one of the coral reefs in the warmer seas of the Antipodes?"

"But what use would Pretorius have for an ancient altar? And what does the skeleton have to do with it? I confess that I find this business baffling. The more we know, the less clear it becomes."

"I believe that the poor man whose bones we found was sacrificed on the stone Pretorius seeks. His head was chopped off, and no doubt his blood was used in some dreadful rite. Just as the fingerbones led us to the skeleton, so the skeleton could lead to the stone. It is, after all, still missing its skull."

"But did not Pretorius say, as a parting shot, that he had already found—"

Brunel broke off because the old fellow who guarded the gate of the yard at night was running down the narrow street toward us, slinging his rattle around his head and yelling murder.

As the watchman hurried off, I asked Brunel what he was planning. He put a match to a lantern, closed the glass on the yellow flame, and said, as much to himself as to me, "The lock of the gate is untouched, so it's quite plain how they came here, and how they left with their prize," and strode off through heaps of construction material toward the riverside edge of the yard.

I caught up with him on the wharf, where the boat with the canvas-covered load rocked on the greasy swell. I said, picking up from his absent-minded remark, "Pretorius saw me enter the tunnel with you. I should have guessed his vantage point after hearing Jake Mullins's tale."

"We always come back to the river," Brunel said. He gave me the lantern and climbed down into the boat and began to undo the rope which lashed the canvas over what was soon revealed to be some kind of boiler: a pair of upright, conjoined cylinders cast from heavy, dull metal. The summer twilight had quite faded from the sky now, and lamps on moored ships and along the far side of the river were twinkling in the dusky blue.

Brunel began to fold up the canvas. "There's no question that he has some need of those bones," he said. "He tried to find them and failed, and then waited for us to haul them up. I am grievously at fault, Carlyle. I should have had the bones taken to a safer place."

"The Tower might have done it," I said, "but anywhere else may not have withstood Dr. Pretorius's determination. Your watchman was lucky that the theft was accomplished before he started his shift; we already know that Pretorius will murder to get what he wants."

Brunel looked up at me. "I am determined to see this affair through to the end, and quickly. I would be grateful of your help, but I will understand if you feel that you have discharged your obligation."

"Would it not be better to go to the police?"

"What would I tell the police? The truth is too fantastic, and anything less would not stir them to any great haste. Yet speed is of the essence now that Pretorius has his prize. He seemed very anxious to get hold of those bones, did he not? I do not think that he will waste time, now he has them."

"Do you propose that we break into his museum, then?"

"That's what he wants us to do. Or at least, that's what he wants you to do. Why else would he have been so careful to tell you where it was? I think he needs you as much as the bones, Mr. Carlyle, and I also think that his choice of the location of his establishment was quite deliberate. It must be somewhere above the grave of this famous stone, for his neighbour mentioned the noise of construction work, which was no doubt the noise of excavation work."

"Dr. Pretorius has been digging down toward the place where the stone is buried."

"Exactly. And that is his Achille's Heel. Ah, here they are at last."

An eager young man in a brown suit, with a bowler hat perched on red curls, was leading two labourers down the wharf. The red-haired man, Roger Withers, was Brunel's assistant in his gaz experiments; the labourers, Thomas and William Dowling, were from the corps d'élite of men who had worked in the frames of the tunnelling shield. Brunel climbed out of the boat and briefly told them what had been stolen from him, and why he thought it important that he get it back. He introduced me as an antiquarian, made no mention of the supernatural part of the story, and concluded by saying that this was dangerous work, and if anyone wanted to jack now he'd think no worse of him.

Thomas Dowling, his vigorous black hair pulled back in a sailor's pigtail, said that it couldn't be worse than working the shield; his brother, a stocky man with a broad, ruddy face framed by muttonchop whiskers, added that nothing could be worse than that. I saw that these rough, uneducated men had a deep respect for Brunel, and would have made a good fist of digging to the centre of the earth if he had proposed it.

"We'll use the Lady Sophia," Brunel told Withers. "Have Thomas and William fetch carbonate of ammonia and sulphuric acid, and let them break out tools they feel most comfortable using. I'll prime her, and if you're not back in five minutes I'll be gone without you. Oh, and bring my pistol, and a bottle of brandy."

As Withers and the two labourers dashed toward the shed, Brunel clambered back into the stern of the boat and began tinkering with the valves and levers of its curious boiler. There was a sharp, distinct chemical smell, and water gurgled through copper pipes that groaned and cracked as they took up their burden.

"I must suppose," I said, after I had climbed down, "that this is one of your gaz engines."

"It is the only gaz engine we have," Brunel said, laying a hand on its pipes. "No doubt you have noticed that it has two condensers. One is warmed by circulation of hot water, and the other is cooled by passing cold water through its tubes. I am running the engine on a closed, inefficient cycle to establish that important differential. At full power, gas expands in the heated condenser and is held in its condensed state in the other, giving a difference of some thirty-five atmospheres. That provides the motive force to drive a longitudinal paddlewheel beneath the stern. If we could but scale it up, we could get a man-of-war up to twenty knots."

"As it is," Withers said, appearing at the edge of the wharf above our heads, "it's a miracle we haven't blown ourselves to kingdom come, and most of Rotherhithe with us. Here are the carboys, Mr. Brunel. I reckon we're about as ready as we'll ever be."

"And my pistol?"

"I have it here, Mr. Brunel. May I say that I am not happy to bring it along."

"We may need the advantage of a little surprise," Brunel said. "Get everything aboard, Mr. Withers, as quickly as you can."

The carboys, cradled in wicker baskets packed with straw, were carefully lowered to the boat. Brunel siphoned heavy, oily acid from one, while Withers scooped gritty white powder from the other into a hopper. The two brothers settled in the bow, packed in tight with an armoury of picks and crowbars, and a long-shanked maul. The pipe-bound double cylinder of the gaz engine began to emit an urgent rattle, and a high-pitched whistle that quickly climbed beyond the range of human hearing. A red needle moved by distinct jerks across the calibrated face of a pressure valve. The oiled brass and steel elbow of the drive-shaft crank lifted and jammed; Brunel whacked it smartly with a spanner and it began to pump smoothly up and down. He took a cigar from a waistcoat pocket and lit it from the lamp, watched the trembling needle creep toward zenith, and at last declared that we were ready to go.

Thomas Dowling cast off at the bow, and Withers cast off at the stern. The little boat thumped against the pilings of the wharf, and then Brunel engaged the drive-shaft and the boat shook itself and shot forward into the main current of the river.

I shall always remember that short voyage upriver. Driven by the gaz engine, the brave little boat cut a fast and sure path against the current. Waves stood at right angles on either side of its bow, and a wide foamy wake beaten by the paddlewheel spread behind, glimmering on the river's black flood. Brunel stood with his cigar jammed in the middle of a broad grin, one hand clapped to the brim of his stovepipe hat, the other on the wheel which was connected by a jointed shaft to the rudder. Darkness was thickening in the air, and long constellations of lights twinkled on either side of us. We passed through the central arch of the five white stone arches of London Bridge, passed beneath the great central span of Southwark Bridge's great ironwork causeway, and overtook a string of barges, quite startling the lighterman at the tiller, who stood up and shouted and waved his cap at us as we sped past. The necklace of gas lights strung along Blackfriars Bridge quickly drew near. At Brunel's instructions, Withers opened a valve, exhausting gas pressure from the condensing cylinders in a series of sharp retorts, and the Lady Sophia's speed dropped to less than a knot.

We cut in close to the embankment, puttered past two paddlewheel boats at their moorings, and turned into a wide recess in the slimy stone wall of the embankment, with the bridge's first arch looming high above us and a great iron grating ahead, half-submerged in the slop of the river.

The two labourers grappled us tight to this portcullis. A dank fetid breeze blew from the darkness beyond it. I could hear, on the road thirty feet above, the sound of horses's hooves and the clatter of cart wheels, and snatches of conversation that rose for a moment above the dull roar of the city. My heart was beating in my throat. I had the strange notion that at any moment a policeman would lean over the wall, and raise hue and cry.

Brunel directed the light of a lantern over the ironwork. His face was flushed with exhilaration, his gaze sharp. He pointed to a joint, and William Dowling applied the tip of his pick and heaved smartly upward. A whole section of the iron grid swung forward, and Brunel and the labourers lifted it above their heads and Withers lashed it tight.

Brunel wiped his hands with a bit of oily cloth and said, "My guess is that this is how Pretorius came and went. He cut this through, bolted on hinges, and wired the whole shut beneath the waterline. You'll notice the cuts are fresh, Mr. Carlyle, and not yet rusted over."

William Dowling was leaning over the side, probing the water with his pick. He reported that he thought we had enough clearance, and we grabbed hold of the sides of the opening and hauled our little boat through. As soon as we had passed beyond the grating, I felt an agitation in my breast pocket. It was the fingerbones, rattling inside their tin. Brunel looked at me when I took it out, his face a pale smear in the gloom, and said, "We must be on the right track."

"Where does this lead?"

"To Hampstead, eventually," Brunel said. "This is the Fleet River, although it is more sewer than river now. Break out the brandy, Mr. Withers. We'll all have a nip to hearten us before we go on."

The brandy bottle was passed around, and we all took our nip, and wet our handkerchiefs and tied them over our noses and mouths against the stench. Adjustments were made to the gaz engine, and we puttered forward at walking pace. Thomas Dowling stood at the bow and held up a lantern, illuminating slimy brick walls that curved up on either side to a ceiling a good thirty feet overhead, heavy stone arches, and a kind of quay or raised path to our left. Water gushed from side channels cut at different heights in the walls, and dripped from the arched ceiling, where a forest of white stalactites clung. The stench thickened, palpable in the black air. The water was flecked with islands of filthy foam. The bloated carcass of a dog bumped against the side of the boat, dipped and whirled, and waltzed away.

Brunel told me that this stygian channel had once been the tidal inlet of a tributary of the Thames which rose in Hampstead and flowed south through Camden and King's Cross. It had marked the western boundary of the city in Roman times—the line of the old wall was to our right. The lower reach, through which we were passing, had been widened and deepened after the Great Fire of London to make a canal with wharves thirty feet wide on either side, but it had quickly fallen into disuse, and a hundred years ago the river between Holborn and Fleet Bridges had been arched over and Fleet Market built on top. The rest of the lower reach had been channelled underground thirty years later, and only a few years ago Fleet Market had been moved, Farringdon Street had been laid out, and the buried river had become no more than a main channel for the area's sewer system. So the living become transmuted and diminish when translated to the realm of the dead, and yet still persist.

We passed between great stone bulwarks: the remains, according to Brunel, of the footings of Fleet Bridge. Farringdon Street was now directly above us. I was quite unable to match our underground thoroughfare with the living street thirty feet above our heads, but as we passed a channel cut through the high kerb on our left, the bones began to rattle even more furiously in their tin. Brunel swung the bow of the Lady Sophia about, and asked the two labourers to check the depth of the channel.

"There's a breeze blowing out of it," Withers said.

"I can feel it too," Brunel said. "What do you feel, Mr. Carlyle?"

"A certain oppression, from the close atmosphere."

"But nothing else?"

"It is curious. This is a very ancient place, and yet—"

William Dowling, who had been leaning out at the bow, sinking his pick here and there in the water, suddenly reared back with a cry. "I saw a face," he said. "Looking up at me out of the water."

"You saw a reflection of this lantern light," his brother said. "Don't mind him, Mr. Brunel. He had a little more than a nip of your fine brandy."

"It was a man pale as snow," William Dowling said. "Very handsome and very horrible at the same time."

"Not your own reflection then," his brother said, "unless you were mistaken about the handsome part."

I insisted on looking, but saw only rippling lines of lantern light moving to and fro like yellow water snakes over the black surface of the thick current. Thomas Dowling retrieved his brother's pick, the handle of which stood up from the surface like the hilt of Excalibur, and reported that there was a good three feet of draft.

"We'll push on, boys," Brunel said, and opened the throttle of the gaz engine. The boat glided through the channel into the arch of the low tunnel beyond. The dripping bricks of the ceiling were only five or six feet above the water, and we had to crouch low. Once, a pipe on top of the gaz engine snagged on something, but the boat shuddered and scraped free. Then the echo of the beat of its drive shaft dropped away, and cooler, slightly fresher air blew in our faces. Thomas Dowling raised his lantern above his head, and Withers held up another; by their double light I saw that we had entered a wide lake under a high ceiling of fan vaulting—the flooded cellar of some ancient building long buried by the accretion of centuries. A rushing stream dropped in a fall of white foam from a narrow channel at the far end, and there was a bank of tumbled stones and clay along the left-hand side. A rowing boat was tied up at the foot of a rough wooden staircase that dropped down to this narrow shore from a ragged opening in the ceiling.

As Brunel steered toward this, the fingerbones beat so strongly in their tin that it jumped from my grasp and fell into the puddle of water at my feet. As I bent to retrieve it, a breeze got up out of the darkness, and the boat began to rock. Little waves ran across the width of the lake and broke in white water on the stones of the banked shore. Spray flew up and dissolved into a thickening mist that rolled over the unrestful water. Brunel looked at me, one eyebrow raised, and I told him that as in the tunnel it was a phenomenon with no cause I could discern.

The bow of the boat bumped against the shore, and the two labourers sprang out, thigh deep in swirling mist, and made us fast. I drew the blade from my cane and clambered out after Brunel, who told Withers to stay with the boat and keep the engine pressured before leading the Dowling brothers and me up the rickety stair.

The bones tick-tick-tickedin the tin, matching the pulse in the base of my throat. We climbed out into a clammy, stone-floored basement, and something toad-like stirred above the door on the far side. I dismissed it in a moment, but before I could raise my own warning the door burst open and half a dozen men crowded through, all of them with shrunken, misshapen heads, all of them armed with pistols.

Dr. Pretorius stepped into the room behind them, his smile triumphant, and bid us welcome.

"Where are his bones?" I said. "And for that matter, who is he, and what do you want with him?"

"All will become clear soon," Dr. Pretorius said, tapping the side of his nose.

The Dowling brothers were tied up and left in the care of two of the shrunken-headed men; Brunel and I were herded back down the stair. Withers was waiting for us at the bottom, sitting on the ground with his hands clasped on his head, the giant savage in his Arabian Nights finery standing watch over him, a pistol in each fist.

"Bring him along," Dr. Pretorius said. "We may be in need of fresh blood." As we picked our way along a narrow path beaten through the rubble, he told me, "It has all worked out very nicely. You saved me the trouble of bringing up the bones, and then you delivered yourself into my hands. There is still time to recant, by the way. Come in with me now, and your reward will be of this world, and not the next. We will do such things as men have only dreamed of."

"I believe you have already had my answer," I said.

"You'll help me anyway," Dr. Pretorius said, "but it would be so much more convenient, and I would be far more forgiving of your friends' trespasses, if you were to give your help freely. Through here, if you please."

A low, irregular opening had been hacked into the tightly mortared stones of the wall at the far end of the lake. Menaced by the pistols of Dr. Pretorius's servants, Brunel, Withers and I scrambled through a passage driven through the earth to a low-ceilinged, stone-floored grotto lit by lanterns that hung from a ceiling of overlapping boards propped up by a forest of stout beams. Shovels and picks lay in a heap in a corner; tall jars of black glass each as big as a hogshead barrel stood in a row along one wall; and something square and waist-high was shrouded in a red and gold Persian rug in the centre.

Withers, his upper arm gripped by the enormous hand of the savage, shivered at my side while Brunel coolly walked around the perimeter, tapping the supports and advising Dr. Pretorius to have them wedged more tightly, before the whole enterprise collapsed upon him.

Dr. Pretorius turned his greedy, exultant smile to me. "He doesn't know anything important, does he? Numbers and angles, cosines and arcs and logarithms, pounds per square inch—" he snapped his long white fingers, dismissing them. "We know, don't we, Mr. Carlyle, that such trivial calculations are of as much use as smoke in manipulating the true nature of the world. It is not matter which is important, but the forms that underlie matter. By mastering those forms, we can master the world, and the world beyond the world too."

He strutted across the grotto, his mop of white curls brushing the ceiling, and ran a hand over the top of one of the squat jars, like a proud mother tousling the hair of a favoured child. "This is a new race of the children of men," he said, "formed by my own artifice and soon to be quickened by the life force I have discovered. A race able to live in both worlds at once, and mediate them directly. You and I have trained long in the matter of the living and the dead, Mr. Carlyle, but my creatures will be able to do all we can, and much more, as easily as breathing. And I will be their god."

"Monsters," Withers said. His face was as pale as milk under his shock of red hair.

"Quite so," Dr. Pretorius said. "A new world of gods and monsters!"

He stepped up to the square form in the centre of the grotto and pulled the rug away, revealing a pediment constructed of heavy blocks of stained limestone and faced with a carving of a man riding a bull, with a familiar tangle of blackened bones draped across its dished top.

"The altar of the temple of the sun-god, Mithras," he said. "Roman soldiers brought the cult to London, and sacrificed bulls to drive back the darkness of the forests around their fledgling city. They believed that the spilled blood of the bull killed by Mithras was the life force from which sprang every kind of plant and animal, and so the blood of their sacrifices has charged this altar with a special potency. At least one man was sacrificed too. His head was buried here, and his body was wrapped in a bull's hide and thrown into the River Fleet. At last, washed to and fro on the tides of many centuries, it came to rest above the path of Mr. Brunel's pitiful little tunnel. You feel its potency, don't you, Mr. Carlyle? Do not deny it—I can see in your face that you do."

The carving on the front of the altar was very similar to the design of the buckle Brunel and I had found with Ulpius Silvanus's skeleton. I saw now that the man was not merely riding the bull: watched by two robed and hooded figures, one holding its torch high, the other low, he was pulling back the bull's head with his left hand while cutting its throat with a long narrow knife held in his right, all this within a ring in which dogs and scorpions and hares and fantastic chimeras chased each other's tails.

As I stared at this carving, only half-hearing Dr. Pretorius's gloating speech, I saw a star kindle deep within the stone, and discovered that I could not look away. The star grew brighter and brighter until with a soundless explosion it burst open like a flower, shining beyond the limits of the stone. I cried out and clapped my hands over my eyes, but the light burned through everything. I could see, past the shadows of my own hand bones, the shadows of the bones within the flesh of the men around me; could see the misshapen homunculi stir in their black glass jars; could see at the heart of the light, like a pupa in its case, a ragged unformed figure jerking back and forth, as if trying to free itself. Then it stilled, and turned its terrible dark gaze toward me.

Brunel told me later that I bellowed like a wounded bull and staggered backward and dropped to my knees, the heels of my hands pressed tight to my eyes. As everyone turned to look at me, he planted his back against the wall and kicked as hard as he could at one of the timbers which supported the roof. It gave with a rending sound and he kicked again and it dropped free, thumping on the stone floor as gravel and stones, and then a cascade of water burst out of the widening hole in the low ceiling. Wherever the water splashed on the stone floor it burst into steam, filling the little grotto with twisting snakes of white vapour, but most of the flood hung in the air as if pouring into an invisible mould: a glassy column that spun faster and faster, with the form of a man becoming dimly visible within it.

I was jolted from my fugue by cold, filthy water washing over my knees, thighs and waist, and staggered to my feet as the flood continued to rise. For a moment, the glassy figure stared straight at me, and then the spinning column burst, drenching everyone and everything. Half the lanterns hung from the ceiling immediately went out; the rest swung crazily, sending shadows swarming around the flooded grotto. Dr. Pretorius was knocked down, his pale hands clawing above swirling water as his servants rushed to his aid, and I grabbed a pick and swung it at the dazzling block of the altar.

I still do not know if the impulse was mine, or if it sprang from the knowledge which had been rammed into my brain.

Black bones smashed; metal rang on stone. Drenched, half-blinded by light only I could see, I swung again and again. I was dimly aware that Withers was beside me, matching me stroke for stroke, and then one of the stones shattered and the burning flower burst like a soap bubble. The flood surged higher and we were all of us knocked down. Hardly aware of where I was, I breathed in a solid gush of water that seared all the way to the bottom of my lungs. Someone grabbed me and hauled me upright, and I saw, beyond a swarm of green and red afterimages, that Dr. Pretorius and his servants were battling an army of snakes made entirely of water. A glassy python surged around the trunk of the giant savage and bore him under; Dr. Pretorius was clinging with one arm to one of the black jars and swiping at darting ropes of water with the other. Then Brunel and Withers dragged me backward, through the half-flooded tunnel.

The lake had risen too. Waves full of a milky light dashed over the bank, washing over our knees. The Lady Sophia swung back and forth at its mooring, banging against the stairway. At the far end of the lake, only the keystone of the arch showed above the restless water.

We had climbed to the top of the tottering stairs when Brunel suddenly damned himself for a fool and swung over the rail. Withers tried to pull him back, but he shook off his assistant's grip, shouted that he would put an end to this and we must save ourselves, and swarmed down the frame of the stairs toward the tossing boat. I was still half-blinded and dizzy; Withers got his shoulder under me and helped me into the basement, where the two brothers were struggling furiously to free themselves of their bonds, and the servants Dr. Pretorius had left to guard them lay in insensible heaps on the stone floor.

"They fell down like unstrung puppets just a moment ago," Thomas Dowling said, as Withers sawed at his ropes with a knife he had found on one of the unconscious servants.

"I think they were intimately connected to Pretorius," I said. "Perhaps he is also unconscious."

Withers said, "Only unconscious? I hope the monster is drowned."

He had cut through the rope binding Thomas Dowling's arms, and had begun to free William, when Brunel appeared at the door, soaked through, wide-eyed, and breathless. "No time for that," he said. "She's set to blow!"

We fled up a flight of stairs and burst through heavy curtains into a big room cluttered with cases and glass jars on stands. Faintly lit by lamplight that filtered through the gold-painted windows, a two-headed baby drowned in oily liquid inside a tall cylindrical jar opened both pairs of eyes and stared at me; at the same moment, the polished wood floor heaved violently and the curtains billowed out and a great gush of black smoke filled the room. Jars tottered and fell and smashed, the windows all fell in, and the floor heaved again and with a great groaning crack split jaggedly down the centre.

Forty or fifty feet below our feet, the gaz engine of the Lady Sophia had exploded like a bomb.

Everything in the museum slid toward the crack, gaining speed, smashing together, and dropping into the smoking abyss. Thomas Dowling hauled his brother up the tilted floor by the scruff of his shirt; I clung to Withers as he helped Brunel knock shards of glass from a windowframe. We piled through, sodden, smoke-begrimed, hatless and breathless. Roof tiles and broken glass smashed on the pavement all around. We picked ourselves up and ran, pursued by a great wave of smoke and plaster dust as the building dropped in on itself floor by floor, plummeting into the great cavity beneath.

No story is ever finished.

I decided to write this account of my adventure with the brilliant young engineer after I read in the Times that an excavation under the direction of Professor W.F. Grimes of the Museum of London had uncovered the remains of a small Roman temple. It had been built in the early part of the third century, and sculptures and a silver incense box buried under the stone floor suggested it had been dedicated to the worship of the Persian god Mithras.

I paid a visit that afternoon. The site was not on Farringdon Street, but within the old walls of the City, near the buried course of the Walbrook River. Raw new buildings of red brick or steel and concrete were springing up everywhere, but the considerable scars left by the war were still much in evidence. New office buildings and brief rows of surviving Victorian buildings stood amidst the rubble fields of bomb sites. Only a few months after the fire storms of the Blitz, these ruins had become fields of wild flowers as seeds buried for centuries woke in the ashes: lupins, poppies, pansies, violets, and, most notably, the yellow flowers of London Rocket.

The site of the temple had been discovered while the foundations for an office block were being dug. The archaeologists worked in plain view, in an oblong terraced pit set at an oblique angle to the street. I confess to being gripped by considerable apprehension as I approached, but if there had ever been ghosts or other revenants at that place, they had long since departed. It was pleasant to lean on my cane in the warm sunlight and watch the crew of young men and women at work amongst a narrow maze of low stone walls set at different levels, uncovering the past inch by inch with trowels and paintbrushes, sifting soil through wire mesh pans, washing finds in rocking trays of water.

I knew, as they did not, that this temple was not the first in London to be dedicated to Mithras. Dr. Pretorius had hunted out the original, and uncovered not only a source of power, but the site of an ancient tragedy. The temple founded by Ulpius Silvanus had stood just outside the boundary of the new city, beyond the western bank of the Fleet river. As the city grew and became more settled and civilized, and more and more soldiers retired from duty at the borders of the Empire to become merchants and traders, the Mithraic cult had become more concerned with fostering the business concerns of its members than with sacrifice, and a sizeable faction had argued that the temple should be resited inside the city, within the safety of the newly built city wall. Ulpius Silvanus had refused to listen to them, and they had murdered him on the altar of the temple he had built.

I had learned all this in the instant I had confronted his ghost, and I had told the story to Brunel and the others as we warmed ourselves with brandy in the long shed at the site of the Thames Tunnel.

"The ghost was bound to the skull buried beneath the altar, mingling with the puissance of the accumulated charge of the sacrifices, but it retained a connection to the rest of the bones, too. It was able to follow the ways of water like a spider at the centre of its web. That is why, I think, I felt no presence in the tunnel."

Brunel favoured me with his sharp, inquisitive gaze. "Did any of Pretorius's boasts have substance, or was he a raving lunatic? I favour the latter, of course."

"Amen," Withers said.

"You did not see what was inside the altar," I said.

"I saw you fall to your knees and cry out," Brunel said. "I saw the floodwater behaving strangely, as if vibrating to some harmonic, and all the rest was smash and flood and every man for himself."

"You freed something," Withers said. He had a blanket draped over his shoulders, and although he was no longer shivering he was hunched into himself, clutching his glass of brandy in both hands. He said, "I felt it pass through me, like a wind."

I said, "The ghost of Ulpius Silvanus has become something more than the ghost of a man, and something less than the ghost of a god."

Brunel nodded, and for a moment we were all lost to each other in our own thoughts. Then he raised his glass and said, "Whatever we believe we saw—or did not see—we have defeated something evil. Let's be content with that."

You know the rest of his story, of course. Work on the Thames Tunnel eventually resumed, and it was completed eleven years later. At Brunel's invitation I attended the opening ceremony and finally met his father; neither of us mentioned the skeleton we had retrieved from the river, or anything of our adventures. By then, Isambard Kingdom Brunel had become the most famous engineer of an age in which engineers were feted as heroes. He built more than a thousand miles of railway. He built bridges and viaducts and tunnels; although he did not live to see it completed, he designed the beautiful suspension bridge over Clifton Gorge. He built three great ships, and the strain of finishing the last of these, the Great Eastern, brought on his early death.

It was at Napier Yard in the Isle of Dogs, in the shadow of his great, doomed ship, that I last saw him. It was a foul, rain-whipped night in late December, some twenty-five years after the affair of Dr. Pretorius and the lost temple. For the last four weeks, Brunel and his men had laboured unsuccessfully to ease the Great Eastern down steel tracks into the river, and now, to almost universal derision, the project was stalled. He was prematurely aged and very weary. He walked with the aid of a stick, and his face was haggard and deeply lined, and spots of hectic colour burned on his cheeks. His former energy showed only in his fierce gaze.

"It's good to know," he said, after we had repaired to the shelter of his office. He had a bad cough, and his voice rasped from an old injury to his throat. "The press is taunting me, I am plagued by the idiotic suggestions from half the cranks in England, my shipbuilder is so bitterly jealous of me, and so bad at managing his business, that he tries to avoid his commitments while at the same time demanding advance payments for work he has not done … So it is good to know that I must deal with a mere engineering problem. I have already ordered more hydraulic presses from the Tangye brothers. They are excellent men, and in the New Year I will have her afloat, I promise you. Unless, of course," he added, with a wry smile, "you could invoke that ghost from our old adventure, and raise such a tide that would float her, cradle and all, straight off the slipway."

"That is beyond such little power I have, I fear," I said.

Brunel insisted on paying my fee. "I would swear that you haven't aged a day since I last saw you," he said, as I drew on my Inverness cape.

"It is one of the few perks of my trade."

"You should—" he said, and was seized by a coughing fit, and had to take a drink of water before he could speak again. "You should teach me that trick. I still have so much to do."

"It would take up all of your life, as it has mine. You would have no time for your great works."

"Pretorius was right, wasn't he? There are two worlds, and you must choose which to inhabit."

"He was wrong about most things," I said, "but he was right about that."

I did not see Brunel again. The Great Eastern was launched in the New Year, but Brunel was still embroiled in disputes with his shipbuilder, who against his advice had been given the contract to outfit the ship. He suffered a massive stroke a few days before the Great Eastern's maiden voyage, and as he lay dying received news that two of the ship's boilers had exploded, destroying a funnel and the grand saloon, and killing six of the crew. His ship was so strongly built that she easily survived the disaster, but for Brunel it was a fatal blow, and he died the same night.

As for Dr. Pretorius, he somehow survived the destruction of the lost temple, and a few years later quit the country for the United States, ahead of a scandal involving a patent electric elixir. At the time I write, he is living in some great style on the Baja Coast in Mexico, having made a fortune treating movie stars at his clinic. Although I admit that I have a faint professional curiosity about how he has lived to such a great age (my own springs naturally from my familiarity with the matter of the dead, I suspect that there is nothing natural about Dr. Pretorius's longevity), but I have no desire to see him again, or to ever attempt to invoke the mingled ghost of Mithras and Ulpius Silvanus. As for myself, I have continued in my trade. I know no other.

Our stories have no proper endings, but are braided into a great unending tapestry, and each of us, living or dead, understands only a little of that grand design.

I was still standing in the sunlight at the edge of the excavation, thinking of the young engineer, and Dr. Pretorius and the lost temple, when one of the archaeologists called to me. She was a slim, pretty young woman in dungarees and wellington boots, her hair tied up in a bright red scarf, her lively face further enlivened by a bold dash of lipstick, her hands on her hips as she looked up at me from the floor of the past. She had mistaken me for a visiting scholar, and wondered if I had business with Professor Grimes. He was away at a meeting, she said, and would not be back until tomorrow.

"It is quite all right," I said. "I'm no more than an idle spectator. There is nothing to trouble me here."