 In a Romanesque crypt that survives below the church, the main altar contains relics of St Thomas of Canterbury.The crypt's 12th-century wall of frescoes depicts the Agnus Dei and symbols of the Four Evangelists, and on the north aisle "St Gerolamo Emiliani introducing orphans to the Virgin" by Jean Francois De Troy, and at the end of the aisle "The Holy Steps and the titular church of Saint Alexius" in wood and stucco by Andrea Bergondi.Connected to the basilica are the buildings of the former monastery, which now belong to the Italian state.