Basilica di San Clemente, Rome

San Clemente is one of Rome's most extraordinary sites—a vertical slice through 2,000 years of history. At street level stands a serene 12th-century basilica with glittering apse mosaics. Descend a staircase and you enter a 4th-century church, buried and forgotten for 700 years until excavated in the 1850s, its walls still bearing early medieval frescoes. Go deeper still and you reach 1st-century Roman streets: the foundations of a Roman house and, beside it, a dark temple of Mithras, complete with its altar showing the god slaying a bull, where a secret male mystery cult once met. An underground spring still rushes audibly through the lowest level. Nowhere else in Rome is the layering of the ages so vividly walkable.