They watched enviously as Rosa led me to a small stone block, the size and shape of Doctor Who’s TARDIS. It had a heavy green door, which she opened with a swipe card. This was the public entrance to the Catacombs of Agrippina, Rosa told me. She palmed a switch, and electric light flooded down from strip lights set in the ceiling.

Steps cut down into the dark. Rosa led the way, her heeled shoes clattering on the worn stone steps. When I pulled the door closed after me, it locked automatically. It immediately felt colder. It was a creepy experience, even in the electric light.

At the bottom of the steps I found myself in a gallery. It was narrow enough for me to reach out and touch both walls, and it was tall, perhaps twenty-five feet high, with an arched roof. The walls were notched, with box-shaped cavities cut deep into them. The light was dim, the electric lamps sparsely scattered. There was more light, in fact, coming from the skylight trenches cut down from the surface.

Rosa gave me the guided tour. “People say they’ve never heard of a Saint Agrippina. There never was one, so far as we know. She was probably just a well-off local matron, sympathetic to the Christians’ cause, who gave them the use of her land . . .”

Burials had given the early Christian community here a problem. Space was always at a premium in Rome. Because of their beliefs the Christians were reluctant to cremate, but land was expensive, even this far out of town. So they began to dig.

“The rock here is tufa,” Rosa said. “Soft, volcanic. It’s easy to work, and it hardens when exposed to the air. And the Romans were used to working underground anyhow. They would dig sewers, waterworks, underground passages for servants to cross from one side of a great villa to another. Many houses would even have a cryptoporticus, an underground recreation area. So when they needed a place to bury their dead the Christians dug, and dug . . .”

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.