Basilica di San Clemente

This place is a layer cake of Roman history, and you can physically walk down through it. The top layer is a twelfth-century basilica with some of the finest medieval mosaics in Rome — a Tree of Life that fills the entire apse, with swirling vines, birds, deer, and tiny human figures, all in vivid gold and colour. It looks like something from an illuminated manuscript blown up to architectural scale. The Cosmatesque floor is original, and the light through the clerestory windows gives the whole space a honeyed warmth.

Go down one level and you are in a fourth-century basilica — the original church, which was buried when the current one was built on top of it. The frescoes down here include some of the oldest surviving depictions of everyday medieval life, and they contain what may be the earliest example of written Italian (as opposed to Latin). In one scene, an angry nobleman yells at his servants using what scholars delicately describe as "colloquial language." The exact phrase translates roughly to "Sons of bitches, pull!"

Go down another level and you are in a first-century Roman house and a Mithraic temple from the second century. Mithraism was a mystery religion popular with Roman soldiers, centred on the ritual slaughter of a bull. The altar stone is still here, carved with an image of Mithras killing the bull. The tricliniums — dining benches — line the walls where initiates would have reclined during ritual meals. You can hear underground water running — the Cloaca Maxima or a branch of it flows somewhere beneath your feet.

Three layers, two thousand years, and the whole thing is run by Irish Dominican friars who have been here since 1667. The combination of medieval mosaics, buried churches, pagan temples, and running water beneath your feet makes this one of the most extraordinary archaeological experiences in Rome, and most tourists walk right past it on their way to the Colosseum, which is two hundred metres away.