Nowhere better illustrates the various stages of Rome's turbulent past than this fascinating multi-layered church. The ground-level 12th-century basilica sits atop a 4th-century church, which, in turn, stands over a 2nd-century pagan temple and a 1st-century Roman house. Beneath everything are foundations dating from the Roman Republic.
The street-level basilica superiore features a marvellous 12th-century apse mosaic depicting the Trionfo della Croce (Triumph of the Cross) and some wonderful 15th-century frescoes by Masolino in the Cappella di Santa Caterina showing a crucifixion scene and episodes from the life of St Catherine.
Steps lead down to the 4th-century basilica inferiore, mostly destroyed by Norman invaders in 1084, but with some faded 11th-century frescoes illustrating the life of San Clement. Follow the steps down another level and you'll come to a 1st-century Roman house and a dark, 2nd-century temple to Mithras, with an altar showing the god slaying a bull. Beneath it all, you can hear the eerie sound of a subterranean river flowing through a Republic-era drain.