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.