The "Liberation of Saint Peter" is an event described in the Acts of the Apostles.

Acts 12:7

"And behold, an angel of the Lord stood next to him, and a light shone in the cell. He struck Peter on the side and woke him, saying, "Get up quickly." And the chains fell off his hands."

Aelia Eudocia, consort of Valentinian II, had received these chains from Iuvenalis, bishop of Jerusalem. She later presented the relics to her daughter Empress Eudoxia (wife of Valentinian III), who finally bestowed the chains to Pope Leo I (aka "Leo the Great". According to the legend, Leo compared them to the chains of St. Peter's imprisonment in Rome - and miraculously the two chains fused together.

The basilica San Pietro in Vincoli (= Saint Peter in Chains) was erected to house these valuable relics. It was consecrated in 439. The basilica underwent several restorations and further work in the eleventh century.

Most people entering the church may not have come to see the chains, but the Tomb of Pope Julius II. But there is not only the statue of Moses, Michelangelo's undisputed master work, but as well this statue of the very relaxed Pope Julius II.