The Basilica is, according to disputed legend, constructed atop the grave of Saint Peter. What is known is that before this site was Piazza San Pietro, it was Nero's Circus, a chariot track like the Circus Maximus. At halftime, the Romans killed Christians for the luls (making them fight gladiators or wild animals or crucifying them, then tarring them and lighting them on fire to provide floodlights for evening events). Peter was one of those killed here (he was crucified). So while it's disputed whether he is buried here, he did die here.