In 1857, the prior of the Basilica 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, fourth-century basilica—with numerous frescoes—but a second-century Mithraic cult temple and a first-century Roman home. Visit the church and descend through these layers of the city’s history.

Via Labicana 95, 39/06-774-0021. Photo by Trigger Image/Alamy. This appeared in the May/June 2011 issue.