According to the Irish Dominicans, who have cared for San Clemente since 1677, the lowest level holds two buildings raised after Nero's fire of 64: a large one that present research suggests was part of the imperial mint, and a Roman house whose lower part was made into a Mithraic shrine around the end of the second century. Christians built their basilica over both in the fourth century. The church keeps the relics of Pope St. Clement, the third successor of St. Peter, beneath its high altar, exposed for veneration on his feast, November 23.
Fr. Mullooly's work also uncovered, in 1863, the tomb of St. Cyril, apostle of the Slavs, who died in Rome in 869. His relics were put in safekeeping during the French Revolution and later lost; a small fragment recovered in the 1960s was placed in the basilica by Pope Paul VI.
Good to know before you go: the excavations are open Monday to Saturday from morning until midday (last entry shortly before) and again in the afternoon (last entry shortly before closing), and on Sundays and holidays from midday into the afternoon. Admission is €10, or €5 for students under 26 with a card; children under 16 with a paying parent, and disabled visitors with a carer, enter free. Daily numbers are limited and the basilica says online booking is the only way to secure entry, so reserve on its website and be at the entrance on Piazza di San Clemente ten minutes before your time. The basilica states plainly that the excavations cannot be accessed in a wheelchair, and photography is not allowed. The upper church is a working church, with Mass in Italian in the morning and in the early evening on weekdays (no evening Mass in July and August). Checked on the basilica's website on October 10, 2026.