Basilica of San Clemente

One of Rome's most extraordinary visits: a 12th-century basilica built directly on top of a 4th-century church, which itself was built on top of a 1st-century Roman house and a Mithraic temple — three layers of history accessible by descending stairs into the underground excavations. The upper basilica has a stunning 12th-century apse mosaic showing the Tree of Life and one of the earliest known examples of vernacular Italian written text (graffiti on a fresco). The 4th-century lower church holds rare frescoes including the famous 'Sisinnius' fresco with the earliest example of vernacular Italian. Down below, a freshwater spring still runs through the 1st-century pagan ruins. Run by Irish Dominicans since 1677.