The Mithraeum under San Clemente

Three churches deep, with a shrine of the Mithras cult at the bottom

Walk into the basilica of San Clemente in Rome and you are in a church built around 1100 to 1120. Go down a level and you are in the 4th century church it was built on top of, holding the largest collection of early medieval wall paintings in Rome after Santa Maria Antiqua, including one of the earliest examples of written Italian: a pagan named Sisinnius swearing at his servants as they haul away a column he thinks is the saint. Go down again and you reach a Roman apartment block, and in its courtyard a mithraeum built around 200 AD, a sanctuary of the mystery cult of Mithras. The main room, about 9.6 metres by 6, was made to feel like a cave, its far end trimmed with pumice, and in the middle stands the altar with its relief of Mithras slaying the bull, still where it was found. It was discovered in 1867 and could not be properly explored until 1914, for lack of drainage. Irish Dominicans have held the basilica since 1667.