We descended another steep staircase and found ourselves in yet another gallery that stretched on out of sight. The corridors branched, one after another, and the walls were all cut with those deep notches.

I was already thoroughly lost, disoriented. We were alone, and the only sounds were our footsteps and Rosa’s gentle voice, softly echoing. The temperature had settled to a mild chill. Around me those open notches gaped like black mouths — and I had no doubt what they had once held. It was an eerie experience.

“The oldest levels are the highest,” she said. “Which makes sense if you think about it. They just kept digging, down and down. They would cut out family vaults, called cubicula, and these niches are called loculi.”

“Niches for the bodies,” I said, my throat hoarse.

“Yes. Wrapped in linen, or perhaps embalmed. Even popes were buried in the Catacombs. But many of the tombs were pillaged in later centuries. Bones were taken away by desecrators, or by seekers of holy relics, or for reburial. Still, some undisturbed tombs were rediscovered over the last few centuries — perhaps there are more still to find. George, this Catacomb alone encompasses fifteen miles, over four levels. And it is estimated that in all the Catacombs some half a million people were buried, over the centuries.”

Like so many numbers associated with ancient Rome, it was a stunning, impossible figure.

“Look.” She pointed to symbols, painted faintly on the walls. The light was kept low to protect the paintwork, it turned out. “Covert Christian symbols, from the days of repression and persecution. The fish you will recognize. The dove, and here the olive branch, symbolizes peace. The anchor implies resurrection. Oh, here is the famous chi-rho, formed of the first two Greek letters of Christ’s name.” It looked like the letters P and X superimposed. “And here—” Carved above one of the loculi, it was like the simple fish symbol, but two fish touched, mouth to mouth, so that it was almost like an infinity.

“What’s that?”

“The symbol of the Order.” I recognized it from my Internet search. She was studying me. “How do you feel?”

“I’m in a two-thousand-year-old graveyard. A little freaked out.”

“You aren’t worried by the enclosure? The narrow walls, the depth — you don’t feel claustrophobic?”

I thought about that. “No.”

“And if I told you I have more to show you yet — that we will go much deeper …”

“Are you setting me some kind of test, Rosa?”

“Yes, I suppose I am. Something about the way we talked in the cafй … You’ve reacted well so far, and I think you’re ready to see more.” She held out her hand. “Will you come? You’re free to go, whenever you like.”

By now I had come to distrust her obviously calculated touching, the overwhelming feelings it evoked in me. But I took her hand again. “What next — open sesame?”

“Something like that.”

We were standing before an innocent-looking niche, empty as the others. It had a two-fish symbol carved over it. Now, to my surprise, Rosa dug out a swipe card and passed it into a slot hidden inside the stone, up behind the fish. I glimpsed a red light, heard the unexpected humming of electronic gear.

And then, with a stony grind, a kind of trapdoor opened up beneath me — and bright light flooded up into the dusty air. I leaned forward to see. There was another staircase, but this was of polished metal, and it led down to a floor of gleaming tiles.

There was a whole room down there — a modern office, I saw. I glimpsed a desk, a girl behind it in a simple white smock, peering up at us. Fluorescent light glared up gray-white but dazzling bright after the gloom of the Catacomb. I was amazed — even stunned. It was the last thing I’d have expected to see; it was hard to believe it was real.

Rosa was grinning. “Welcome to my underground lair, Austin Powers.”

“Not funny,” I snapped.

“Oh, lighten up.” She turned and descended. I followed.

And so I entered the Crypt for the first time.

The receptionist sat behind her wide marble desk, smiling at us. I glimpsed a winking rack of small TV monitors behind the surface of the desk, and one red-eyed camera peered directly at me from a wall. It was all quite normal, electric bright, certainly not as chill as the Catacombs. But there was no daylight, of course, not a scrap; it reminded me how far I was underground.

“We don’t use this entrance much,” said Rosa. “There are many ways in, from our shops and offices on the surface — most of them in the suburbs to the west of the Appian Way — though we have a couple of routes that lead to the center of the city. But I wanted to bring you this way. It is the oldest.” She smiled, almost mischievously. “I suppose I wanted to put on a show … Are you okay?”

I simply had no idea what to expect. “I’ve never been in a convent before,” I said.

“You aren’t in one now. Come on.”

We walked toward the wall of the anteroom. Automatic doors slid out of sight. We stepped into a corridor, just as brightly lit as the anteroom.

The corridor curved out of sight. It was my first impression of the true size of the place. For sure it was a hell of a lot bigger than the anteroom.

And the corridor was full of people: a great murmuring crowd, deep underground.

There must have been hundreds, just in that first glimpse. The human traffic in that corridor was as dense as Oxford Circus on a summer Saturday, or Times Square at New Year. Most of them were women. Many were in street clothes, but some wore a kind of uniform, a simple white dress or trouser suit with sewn-in threads of purple. They walked in neat files, passing in and out of the rooms that branched off the corridor.

Then there was the smell : not an unpleasant smell, not a locker-room stink, but there was something animal in the air, something potent. The air was hot, humid, and noisy; I found myself breathing hard, dragging for breath.

All this concealed far underground, under that sleepy tourist-trap park.

Nobody seemed aware of anything strange, nobody but me. It was all I could do to keep from staggering back, into the relative calm of the anteroom.

Rosa was leaning toward me. “Don’t let it get to you. I know how you feel. But it’s always like this here. Come on …” Holding my hand, she pulled me forward, and we waded into the streams of people.

Suddenly I was surrounded by faces, all young, many smiling, few showing curiosity at this big sweating Englishman who had been thrust in among them. They all seemed to be talking, and the hubbub battered me, like a wind. But they parted around us, accepted us into the flow.

We passed offices with desks and partitioned cubicles, potted plants and coffee machines. They all seemed very mundane, if crowded and noisy compared to most offices I’d seen, almost as crowded as the corridors. In some places there were copies of the kissing-fish infinity symbol of the Order, done out in chrome strips and fixed to marble walls. All very corporate.

On many of the walls slogans had been incised — in places crudely, by hand, and in others more professionally. They were in Latin, which I can’t read. I tried to memorize them, meaning to ask Peter about them later; there seemed to be three key phrases.

Rosa said that the varying sizes of rooms had names, in the Order’s peculiar internal language — basically modern Italian, I would learn, but laced with terms derived from Latin, and other sources I didn’t recognize. The room names seemed to be a macabre joke, a remembrance of the Crypt’s origin. The largest vaults of all were called cubicula, like the family tombs in the Catacombs, the next largest arcosolia, like the large tombs of the wealthy and the popes — and the smallest of all loculi, like the lonely niche-graves of the poor.