The Catacombs of San Callisto are more than ten kilometres of galleries cut into soft volcanic rock south of Rome, on as many as five levels, holding the graves of many thousands of early Christians. Roman law forbade burial inside the city, so every community buried outside it, and Christians dug downward instead of outward because land along the road was expensive and they wanted to stay together. The galleries are narrow corridors with rectangular slots cut in the walls from floor to ceiling, each one closed with a slab of marble or tile. Most are open and empty now.
The crypt at the centre held the bishops of Rome through the third century. Nine of them were buried in this one small chamber, and five of their name slabs are still fixed to the wall, cut in Greek because that was still the language of the Roman church. The bones themselves were taken away from the eighth century onward and moved into churches inside the walls, where they were safer and easier to visit. What is left underground is the architecture of a cemetery: the shelves, the inscriptions, the scratched drawings, and no bodies at all.