Excavations carried out between 1939 and 1950 on the initiative of Pope Pius XII unearthed a pagan necropolis that the emperor Constantine had filled in to build the early basilica. This ancient cemetery comprises two rows of tombs (dated from the 1C to 4C) and, separated by the so-called "red wall" from a small 2C edifice, called Trophy of Gaius, under which the tomb of St Peter was discovered. One of the tombs encloses Christian mosaics from earlier than the 4C.