And few of them talked. Oddly, it took me a while to notice that. But there was little of the incessant chatter of Level 1. Here, there were words exchanged, brief conversations, but no hubbub. It was, I thought, as if words weren’t really needed here.

It struck me that I had already come deeper than most members of the public, like the schoolchildren in their classrooms above, would ever reach. Nobody would see this — nobody but another member of the Order — and to her, who had grown up with it, none of this would seem strange at all.

And the air was thicker, warmer, and more redolent with that earthy animal musk. I soon felt breathless; my chest strained as I breathed, my lungs ached. After a time I began to feel sleepy, and I had a vague sense of the Crypt and its inhabitants swimming past me, as if I were in a waking dream.

I struggled to think.

“Most people born in the Crypt stay here. Is that true?”

“Not all,” Rosa said. She talked about how people would be sent “outside,” for a day, a month, even years, during their education, or as part of their work. “Like Lucia. And some leave for good, like our own grandmother—”

“Grannie?”

“She was born here, but died in Manchester. You didn’t know that, did you? How do you think families like ours, branches of Regina’s ancient clan, end up in England, or America, or elsewhere? Of course people leave — they always have — sometimes for good. But they retain a loyalty to the Order.” She smiled. “It’s our shared heritage, after all.”

There was a lot I never learned about the Order — how they allocated their children surnames, for instance. I wondered vaguely how babies born here were formally registered, whether the Order had some tame functionary in Rome’s registry of births and deaths. Perhaps some of them grew up here without any official record at all, never leaving the Crypt, living and dying invisible to the state.

Rosa brought me to a kind of open oven, a brick-lined niche in the wall taller than I was. I lingered, curious. A wide chimney snaked up out of sight, caked with soot. There was no fire burning. This was very old — I recognized narrow Empire-era red bricks, the thick-packed mortar between them.

“So what’s this, a barbecue?”

Rosa smiled. “Part of our ventilation system. Or it was. Nowadays we have modern air-conditioning equipment — ducts, pumps, fans, dehumidifiers, even carbon dioxide scrubbers. But that kind of gear has only become available in the last few decades or so. In the eighteenth century we did buy one of James Watt’s first steam engines, but that was deinstalled and sold off to a museum long ago … The very first builders, digging down from the Catacombs, adapted techniques used in the deep mines. You’d light a fire under here and keep it burning, all day.” She pointed upward. “The smoke and heat would rise up the chimney — and, rising, would draw air through the Crypt. Elsewhere there are vents to allow more air in. Sometimes they would work bellows.”

“So you’d get a circulation. Ingenious.”

“In case of problems with the airflow, the Order members would have to block a corridor or passageway.”

“With what?”

“With their bodies, of course. They’d just run to where the problem was. It sounds crude, but it worked very well.”

“Who told them what to do?”

She seemed puzzled by the question. “Why, nobody. You’d look around, see there was a problem, follow your neighbors, do what they were doing. Just as if you were putting out a fire. You don’t need to be told to do that, do you? You just do it …”

I had no reason to disbelieve her. I had still seen no senior management floor, no corner offices for the Big Cheeses, no evidence of a chain of command. Evidently things just worked, the way Rosa had described.

She was still talking about the ventilation system. She slapped the solid brick wall. “The old infrastructure is still there. Always will be. Even now — though we have our own generators, like a hospital, independent of the public supply — we prepare for power outages. The youngsters are still taught about the old ways, given simple drills. If the very worst happened, we could always revert to the old methods.”

I peered at the ancient brickwork. “The very worst? …”

“If everything fell apart. If there was no more power at all.”

That took me aback. “You’re planning for how to keep the air supply functioning, even if civilization falls. ”

“Not planning, exactly. There are few plans here, George. But — well, we’re here for the long term. And you read Regina’s story. Civilizations do sometimes inconveniently fall …”

We walked on, down the crowded corridor. She led me away from the ancient hearth, down further staircases — some of them metal, some of them older, cut from the stone itself — staircases that led ever deeper into the core of the Crypt.

We reached Level 3, the deepest downbelow of all.

Many of the walls here were bare rock, polished so smooth by the passage of bodies they gleamed. It was clearly very old — and yet this level, the deepest, was paradoxically the youngest of this great inverted city.

The corridors here were still narrower than those above. They branched as we walked, until once again I had completely lost my bearings. The air was hot, moist, and thick, and was at first suffocatingly hard to breathe. Carbon dioxide is heavy, I remembered dimly, and it must pool, here at the bottom of the great chamber of the Crypt. But as my body became accustomed to the conditions the viselike pain that gripped my chest eased.

And everywhere we walked we had to push through the endless crowds, the smoky gray eyes huge in the gloom. Nobody spoke, here on Level 3. They just moved wordlessly around each other, on their way through the endlessly branching corridors. The only sounds were the rustle of their soft-soled shoes on the rocky floor, and the steady flow of their breathing — and even that, it seemed to me, was synchronized, coming in overlapping waves of whispers, like the lapping of an invisible ocean. I had the sense of these soft, rounded little creatures all around me, in the corridors that stretched off into the darkness every direction I looked, and of thousands more in the tremendous airy superstructure of galleries and corridors and chambers above me.

As I describe it now it sounds oppressive, claustrophobic. But it did not feel like that at the time.

Rosa seemed to sense that. “You belong here, George,” she said softly. “I’m your sister, remember. If it’s good enough for me … Can’t you feel the calm in here? …”

I felt the need to cut through this odd seduction. “What about sex?”

“What about it?”

“You’ve a lot of young people here, cooped up together. There must be love affairs — casual flings—” I felt awkward; trying to discuss such issues with my long-lost sister, I was reaching for fifties euphemisms. “Do you let people screw?”

She stared at me in icy disapproval. “First of all,” she said, “it isn’t a question of ‘letting’ people do anything. There is nobody to ‘let’ you, or to stop you come to that. People just know how to behave.”

“How? Who teaches them?”

“Who teaches you to breathe? … And anyhow it tends not to be an issue. Most of the men here are gay. Or don’t have an inclination either way. Others usually leave.” She said this as if it were the most usual setup in the world.

It might fit in. Peter, in his long, rambling analyses of what we had learned about the Order, had speculated it might be some kind of heredity cult. Like neuter women, like Pina, gay men could help out with the raising of the fecund ones, the Lucias. But neither class would be any threat to the precious gene pool, because they didn’t contribute to it.