The charming church of San Clemente is a 3D timeline.
There's something for (almost) everyone in this multi-layered basilica run by Irish Dominican monks. From the 12th-century building (with some Baroque touches added later) you see at street level, you can descend through the church's 3D timeline: first comes the 4th-century church, then, below an Ancient Roman street, complete with the rushing noise of the city's cloaca maxima (great sewer) and a temple to the Persian god Mithras.
The 4th-century basilica was razed by rampaging Normans who sacked Rome in 1084: this corner of town suffered the worst damage of all, and not much survived beyond the beautiful choir stalls which now stand in the upper church. The upper church has a wonderful 12th-century mosaic in the apse and frescoes of St Catherine of Alexandria (she who gave her name to the Catherine Wheel) by Masolino (1430).
Downstairs, supports for the upper church disturb the layout of the 4th-century building, but you can still make out the faded frescoes showing scenes from the life of St Clement, a 1st-century Roman bishop who was exiled by the Emperor Trajan to the Crimea.
But it's on the bottom level, where the great sewer swishes by, that you really get a feel for the ancient world. In one insula – an apartment block – some rooms were given over to the worship of Mithras: you can see a fresco in which the god is killing a bull. Across the narrow lane, another housed a titulus, an early Christian meeting place.