Basilica of San Clemente site features

San Clemente is three buildings stacked on each other, and you walk down through them. Each floor is the ceiling of the one beneath, which is the whole reason to come: the site is a vertical section through about nineteen centuries, and the stair does the explaining.

At street level it is a twelfth-century basilica, and it feels complete rather than old. It was rebuilt in one campaign by Cardinal Anastasius from about 1099, so it reads as a single design. The apse is covered in mosaic above the episcopal seat; a twelfth-century schola cantorum — a marble enclosure for the singers — stands in the nave and incorporates marble elements from the original basilica; the chancel is sixth-century marble brought up from the church below. There are early Renaissance frescoes too, and in the atrium a plaque affixed by Clement XI in 1715.

One flight down, the light goes and the walls become painting. This is a fourth-century church, lower and heavier, carrying medieval frescoes of the eighth to the eleventh century. Four of the largest were paid for by a lay couple, Beno de Rapiza and Maria Macellaria, in the last third of the eleventh century — private patrons, named on the wall.

Below that the architecture stops being a church at all. You reach an industrial building, probably the imperial mint of Rome from the late first century A.D. — identified because a similar building appears on a surviving fragment of the city's marble plan. In the fourth century its lower level was filled in with dirt and rubble and its second floor remodelled, which is why the church above sits where it does.

And cut into that is a cave that is not a cave. The Mithraic speleum — the word means cave — is the main cult room, about 9.6 metres long and 6 metres wide: a vaulted rectangle with benches down the long sides and an altar at the end. It was discovered in 1867.

Water runs somewhere below, and you hear it before you work out where it is. At the bottom of the sequence the sound is constant, which is the thing most visitors remember.

The site was found by the people who still own it. Irish Dominicans have held the basilica and the surrounding complex since 1667, and the archaeological traces were discovered in the 1860s by Joseph Mullooly, Prior of their house here.

Practicalities worth knowing at the door: the entrance is on Piazza San Clemente, a five-minute walk from the Colosseum; hours are Monday to Saturday 9.00–12.30 with last entrance at 12.00, and 14.00–18.00; online booking is required to enter the excavations; and the full adult ticket is EUR 10, with EUR 5 for school groups and students under 26 holding a student card.

The apse mosaic, Triumph of the Cross

Twelfth century, in the upper basilica, and a high point of Roman medieval mosaic. It is free to look at: it is in the church, not the excavations.

The Mithraeum

A temple of Mithras on the lowest level, its cult room about 9.6 metres by 6, discovered in 1867 and not properly investigated until 1914 because the site could not be drained. Mithraic temples are deliberately cave-like, and this one is beneath two churches, which is a juxtaposition no reconstruction could improve on.

The frescoes of the lower basilica

Dating from the eighth to the eleventh centuries — among the finest medieval frescoes anywhere. Four of the largest were paid for in the last third of the eleventh century by a lay couple, Beno de Rapiza and Maria Macellaria, and show the life, miracles and translation of St Clement.

The schola cantorum

The twelfth-century marble enclosure in the nave of the upper church, built using elements salvaged from the basilica now buried beneath it — the older building furnishing the newer one.

The Roman alley and the mint

A first-century street you can walk, between a large public building — probably the imperial mint, remodelled under the Flavians — and the Mithraeum opposite. The scale is domestic and the effect is not.

Clement XI's plaque of 1715,

In the atrium, praising the basilica: the moment the church's reputation was formally noticed, four years before it was given its present face.