Complex sentence: Catholic tradition holds that the Basilica is the burial site of Saint Peter, one of Christ's Apostles and also the first Pope; supposedly, Saint Peter's tomb is directly below the high altar of the Basilica.
Split into simpler sentences: Catholic tradition holds that the Basilica is the burial site of Saint Peter, one of Christ's Apostles and also the first Pope. Supposedly, Saint Peter's tomb is directly below the high altar of the Basilica.