The Mithraeum under San Clemente

Three churches deep, with a shrine of the Mithras cult at the bottom

The basilica of San Clemente stands about 300 metres from the Colosseum on the road to the Lateran, in the valley between the Caelian and Oppian hills, and it is three buildings deep. In 1857 Father Joseph Mullooly, prior of the Irish Dominicans who keep the church, realised the ground had once lain much lower, dug beneath the floor and found a whole fourth-century basilica. Under that he found Roman buildings of the first century. Water flooded the lowest level until Father Louis Nolan drove a drainage tunnel through to the Colosseum in 1912-14, and that work exposed buildings burned in Nero's fire of AD 64.

In the first century the street here ran some 20 metres below today's. After the fire the ruins were filled with earth and built over, and two buildings rose on either side of a narrow lane. On one side stood a monumental storehouse of tufa blocks capped with travertine, probably part of the imperial mint; on the other, a brick house at least three storeys high, around a courtyard. Around the end of the second century followers of Mithras took over its half-buried gallery and vaulted summer dining room. Their main hall, about 9.6 metres long and 6 wide, was trimmed with pumice at the far end to feel more like a cave. At its centre stands an altar shaped like a sarcophagus and carved with Mithras slaying the bull, the torchbearers Cautes and Cautopates on its sides; its donor was a pater named Cnaeus Arrius Claudianus. A bust of the sun god Sol and a figure of Mithras born from the rock were found with it, and all three are still in place. A side room held a pit for the leftovers of the cult meal.

In the fourth century the house was filled in and a basilica rose over it, its apse set above the Mithraic rooms; Saint Jerome wrote of the church of Saint Clement in 392. Its walls gathered frescoes from the eighth century to the eleventh. Among the last is the legend of Sisinnius, a pagan who believes his servants have seized Saint Clement when they are hauling a column, and who curses them across the wall in some of the earliest written Italian. The saint answers in Latin. By about 1100 the street level had risen so far that the old church was filled with rubble to the tops of its columns and a new one built on it, the basilica with its twelfth-century apse mosaic and Cosmatesque floor.

The excavations are open to visitors, though entry is by online booking only, with a daily limit.