“The basilica was first built in the middle of the 5th century to house the relic of the chains that bound Saint Peter while imprisoned in Jerusalem.
According to legend, when Empress Eudoxia (wife of Emperor Valentinian III) gave Pope Leo I the chains as a gift, he compared them to the chains of St. Peter’s first imprisonment in the Mamertine Prison in Rome and the two chains miraculously fused together.
The chains are kept in a reliquary under the main altar in the basilica.
The basilica has undergone several restorations and rebuildings, including a restoration by Pope Adrian I, a rebuilding by Pope Sixtus IV and another by Pope Julius II. There was also a renovation in 1875. Some modernizations were made at that time.
Michelangelo’s Moses, which dates from 1515, is the most notable piece of artwork in the basilica. Originally intended as part of a 40-statue funeral monument for Pope Julius II, “Moses” became the Pope’s funeral monument and tomb in his family’s church.”
reference: Sacred Destinations