Why was Saint Peter crucified upside down?

Saint Peter requested that he be crucified upside down because he felt he wasn't worthy to be crucified in the same way as Jesus. It is believed that his body is buried beneath St. Peter's Basilica in Rome.

The story of Saint Peter's crucifixion is told in the Bible, but it does not mention how the act was carried out. However, there was testimony from sources who claimed he was crucified upside down. In chapter 21 of John, it says, "I assure you: When you were young you would tie your belt and walk wherever you wanted. But when you grow older, you will stretch out your hands and someone else will tie you and carry you where you don't want to go." This is taken as proof that Saint Peter was crucified.