In 1857, the prior of the Basilica di San Clemente thought there might be something underneath his 12th-century church, already renowned for its relics and striking mosaic of the Crucifixion. When he excavated, he found not only the original, 4th-century basilica—with numerous frescoes—but a 2nd-century Mithraic cult temple and a 1st-century Roman home. Visit the church and descend through these layers of the city’s history.