This church is one of the most extraordinary archaeological sites in Rome. San Clemente is a 12th-century church built on top of a 4th-century church, which in turn was built over a 2nd-century pagan temple to the god Mithras. Little of the temple remains, but the 4th-century church is largely intact, perhaps because it wasn't unearthed until the 19th century. (It was discovered by Irish Dominican monks; members of the order still live in the adjacent monastery.) The upper church, which you enter from street level, holds a beautiful early-12th-century mosaic showing a cross on a gold background, surrounded by swirling green acanthus leaves teeming with little scenes of everyday life. The marble choir screens, salvaged from the 4th-century church, are decorated with early Christian symbols: doves, vines, and fish. In the left aisle is the Castiglioni chapel, holding frescos painted around 1400 by the Florentine artist Masolino da Panicale (1383-1440), a key figure in the introduction of realism and one-point perspective into Renaissance painting. Note the large Crucifixion and scenes from the lives of Sts. Catherine, Ambrose, and Christopher, plus an Annunciation (over the entrance). Before you leave the upper church, take a look at the pretty cloister -- evening concerts are held here in summer.

From the right aisle, stairs lead down to the remains of the 4th-century church, which was active until 1084, when it was damaged beyond repair during a siege of the neighborhood by the Norman prince Robert Guiscard. The vestibule is decorated with marble fragments found during the excavations (which are still under way), and in the nave are colorful 11th-century frescoes depicting stories from the life of St. Clement. Another level down is the Mythraeum, a shrine dedicated to the god Mithras, whose cult spread from Persia and gained a hold in Rome during the 2nd and 3rd centuries. COST: EUR3.