The Basilica di San Clemente al Lateranois a medieval church which has an extremely long history. Although the church itself is beautiful, the real treasure is to be found underground, in the excavations below. You’ll be able to see the Roman villa that originally stood on the site and had been used as a Mithraeum (a temple to Mithras), plus the 4th-century Christian basilica that predates the upper church and houses ancient frescoes.