One of the most impressive 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 nave is the Castiglioni chapel, holding frescoes 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 nave, 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 area 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.

From the left nave follow the signs to 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 AD. Mithras was believed to have been born in a cave and was thus worshipped in underground, cavernous chambers, where initiates into the all-male cult would share a meal while reclining on stone couches. Most such pagan shrines in Rome were destroyed by Christians, who often built churches over their remains, as happened here. The Mithraeum was installed sometime in the 2nd or 3rd century in the basement of a Roman domus (house). Corridors lead to other of the house's chambers, partly decorated with stucco and frescoes. Proceed through the domus basement, cross a very narrow alley, and enter an adjacent building, the oldest known structure on the property. It was possibly once a mint, gutted by Nero's fire in AD 64 and rebuilt as a courtyard surrounded by small rooms. The herringbone brickwork of the floors and the waterproof plaster seem to indicate that the rooms were used as a depot. Following the signs will bring you back to the lower church.