A favourite with kids for its dungeon-like underground level, this 12th-century basilica is a three-dimensional Roman time-line, a church above a church above an even older Imperial building - a full 18m (60ft) of Roman life separates the earliest structure from the one we see today.
In 1857 the Irish Dominicans - who have run the church since the 17th century - began digs that unearthed the church's fourth-century predecessor, and, beneath, an early Christian titulus (meeting place). The fourth-century structure was razed in the Norman sack of 1084, but the schola cantorum (choir), with its exquisite carving and mosaic decorations, survived and was moved upstairs to the new church, where it still stands.
Also in the upper church is the 12th-century mosaic in the apse, still in Byzantine style but with a theological complexity unusual for its period. Against a gold backdrop, cobalt blues, deep reds and multi-hued greens make up the crucified Christ. From the drops of Christ's blood springs the vine representing the Church, which swirls around peasants at their daily tasks, Doctors of the Church spreading the divine word and a host of animals. Above the cross, the hand of God links heaven and earth, while below, sheep represent Christ and the 12 apostles. The Latin inscription above the sheep says 'I am the vine, you are the branches'. Towards the back of the church, in the chapel of St Catherine of Alexandria, a series of frescoes by Masolino (c1430), possibly with help from Masaccio, depict the life of the saint - she is shown calmly praying as her torturers prepare the wheel to which she was strapped and stretched to death (and for which the firework was later named) - as well as Christ on the cross in between the two thieves.
From the sacristy, steps lead down to the fourth-century basilica, the space broken up by walls supporting the structure above. Fading frescoes show scenes from the life of St Clement, the fourth pope, exiled to the Crimea by Emperor Trajan. Even in exile, he didn't give up proselytising and so was hurled into the Black Sea, tied to an anchor. A year later, the sea receded, revealing a tomb containing Clement's body. After that, the sea would recede once a year and another miracle would occur.
Near the far end of the underground basilica is a modern shrine to St Cyril; the inventor of Cyrillic script was a great figure of the Orthodox churches and credited with bringing Clement's body to Rome.
A stairway leads down to an ancient Roman alley. On one side are the remains of a second-century Roman insula, or apartment building, containing a site where the Persian god Mithras was worshipped. There are three rooms devoted to the cult, which was as popular as Christianity in the late Imperial age. Persian mythology said that Mithras was born of a virgin who was called 'mother of God'… Sound familiar? In the sanctuary, a fresco depicts the god killing a bull. On the other side of the lane are the ground-floor rooms of a Roman house used by early Christians as a titulus. At one of the turns in this warren of antiquity, you can hear water rushing through an ancient sewer on its way down to the Tiber.