I did notice the arched brick vault then. And for the first time I realised that the reddish “dirt” on the floor, several inches deep in places, was detritus from the crumbling bricks and mortar falling from that vaulted ceiling.

“This was a Christian catacombs,” repeated Dickens. “Installed directly under the chapel above.”

“But there is no chapel above,” I whispered.

“Not for many years,” agreed Dickens, rising and trying to flick the dirt from his gloves while still holding the lantern and his stick. “But there was long ago. A monastery chapel would be my guess. Part of the Monastery of the Church of Saint Ghastly Grim’s.”

“You made that up,” I said accusingly.

Dickens looked at me oddly. “Of course I did,” he said. “Shall we move on?”

I hadn’t liked standing in the dark corridor with no light behind me, so I was grateful when Dickens emerged from the niche and prepared to press on. But first he shone the light back into the vault again, passing its beam over the rows and columns of coffins stacked behind the rusting grille.

“I neglected to mention,” he said softly, “that as with their Roman originals, these burial niches are called loculi. Each loculus is reserved for a family or perhaps for members of a specific order of monks over many decades. The Romans tended to excavate their catacombs logically, all at one time, but these later Christian tunnels were dug out over a much longer period of time and tend to stray and wander. Do you know Garraway’s Coffee House?”

“On Exchange Alley?” I said. “Cornhill? But of course. I’ve had coffee there many a time while waiting for a sale to begin in the adjoining auction house.”

“There is a similar old monastery crypt under Garraway’s,” said Dickens, whispering now as if he were afraid some spectral form had joined us. “I have been in it, down there among the port wine. I have often wondered if Garraway’s is taking pity on the mouldy men who wait in its public-room all their lives by giving them that cool crypt down below to hold the rest of those gone missing from what fools call ‘real life’ up there on the surface.” He glanced at me. “Of course, my dear Wilkie, the catacombs of Paris—and you have been there, I know, since I took you there—the catacombs of Paris would not be large enough to hold the rest of the truly missing souls of London if we were all forced to go below, out of the light, down into the mouldy dark where we belong when we forget how to live well among upright men.”

“Dickens, what in the deuce are you going on about…” I stopped. There had been a stirring or footstep down the dark corridor, out of the weak glow of our single small lamp.

Dickens turned the bullseye but there was nothing in the cone of light but stone and shadows. The roof of the main passageway was flat stone, not arched brick. It went on for at least fifty yards. Dickens led the way down this corridor, pausing only to shine his beam in some of the niches that opened to the left and right of the passage. They were all loculi, niches holding stacks of massive coffins behind identical rusted-iron grilles. At the end of the passage Dickens passed his beam of light over the wall and even ran his free hand over the stone, pressing here and there as if searching for some spring-lever and secret passage. None opened to us.

“So…” I began. What was I going to say? You see? There’s no Undertown after all. No Mr Drood down here. Are you satisfied? Let us go home, please, Dickens, I need to take my laudanum. I said, “This seems to be all there is.”

“Not at all,” said Dickens. “Did you see that candle on the wall?”

I had not. We walked back to the next-to-last loculus and Dickens aimed the bullseye higher. It was there in a niche, a thick tallow candle burned to a stub.

“Left by the ancient Christians, perhaps?” I said.

“I believe not,” Dickens said drily. “Light it, please, my dear Wilkie. And walk ahead of me back towards the entrance.”

“Why?” I asked, but when he did not answer, I reached for the candle, fumbled matches out of my left pocket—the absurdly heavy pistol still weighed down my jacket on the right—and lit the thing. Dickens nodded, rather brusquely I thought, and I held the stub of candle in front of me as I walked slowly back the way we had come.

“There!” cried Dickens when we had covered about half the distance.

“What?”

“Didn’t you see the candle flame flicker, Wilkie?”

If I had, it hadn’t registered, but I said, “Just a draught from the entrance stairs, no doubt.”

“I think not,” said Dickens. His emphasis on the negative every time I spoke was beginning to annoy me.

Using his lantern, Dickens peered into the loculus on our left and then into the one on our right. “Ahhh!” he said.

Still holding the slightly flickering candle, I peered into the niche but saw nothing to evoke such an ejaculation of surprise and satisfaction.

“On the floor,” said Dickens.

I realised that the red dust there had been trod down into a sort of path that led to the iron grille and the coffins. “Some recent interment?” I said.

“I seriously doubt it,” said Dickens, continuing his string of negative assessments of my contributions. He led the way into the arched vault, handed me the lantern, and shook the iron grille with both gloved hands.

A section of the grille—its joints and edges and hinges invisible from even a few feet away—swung inward towards the stacks of coffins.

Dickens went through without a pause. In a second his lantern seemed to sink into the red dust beneath him. It took me a minute to realise that there were steps back there and that Dickens was descending them.

“Come along, Wilkie,” echoed the writer’s voice.

I hesitated. I had the candle. I had the pistol. I could be back at the base of the steps in thirty seconds and up them and out into the crypt above—under Detective Hatchery’s protection again—thirty seconds after that.

“Wilkie!” The lantern and the author were both out of sight now. I could see the brick ceiling still illuminated above the place where he had disappeared. I looked back towards the dark entrance to the loculus, then at the heavy coffins stacked atop their biers on either side of the path in the red dust, and then back towards the opening again.

“Wilkie, please hurry now. And snuff the candle but do bring it along. This bullseye does not have unlimited fuel.”

I walked through the open grille door and past the coffins and towards the still-not-visible stairs.

The stairway was of unsteady stone, the narrow vault ceiling of brick, and within a few minutes we had come out into another level of corridor and loculi.

“More crypts,” I said.

“Older here,” Dickens whispered. “Notice that this corridor curves, Wilkie. And the ceiling is much lower here. And the entrances to these loculi have been bricked up, which reminds me of a story by the late Mr Poe of whom I was speaking somewhat earlier.”

I did not ask Dickens to share the story. I was about to ask him why he was whispering when he whispered over his shoulder, “Do you see the glow ahead?”

At first I did not because of the bullseye lantern’s glare, but then I did. It was very dim and appeared to originate somewhere around the bend in the stone corridor.

Dickens lowered the shield over most of the bullseye’s lens and gestured for me to follow him. The paving stones on this lower, older level of the catacombs were uneven, and several times I had to use my stick to brace myself from falling. Just around the bend in the corridor, more main passageways branched to the right and left.

“Is this a Roman catacomb?” I whispered.

Dickens shook his top-hatted head, but I felt it was more to quiet me than to answer me. He pointed to the passage on the right from which the glow seemed to be coming.