San Clemente: three layers of Rome under one roof

The church at street level was built just before the year 1100. It is one of the best-kept medieval interiors in the city: a marble choir, a floor of coloured stone, and in the apse a twelfth-century mosaic of the cross as a tree of life, its vine scrolls filling the whole gold vault.

Under it is the church it replaced, a basilica of the fourth century, filled with rubble and forgotten until an Irish Dominican prior, Joseph Mullooly, began digging in the nineteenth century. Its walls still carry frescoes, one of them with some of the earliest written Italian.

Under that is the first century. You walk down a second stair into a narrow Roman alley between a large public building and a house, and in the house is a mithraeum, a small sanctuary of the god Mithras made around the year 200, with its altar in place. Somewhere close by, behind the walls, you can hear water running.