The Vatican Necropolis lies under the Vatican City, at depths varying between 5–12 metres below Saint Peter's Basilica. The Vatican sponsored archaeological excavations under Saint Peter's in the years 1940–1949 which revealed parts of a necropolis dating to Imperial times. The work was undertaken at the request of Pope Pius XI who wished to be buried as close as possible to Peter the Apostle. It is also home to the Tomb of the Julii, which has been dated to the third or fourth century. The necropolis was not originally one of the Catacombs of Rome, but an open-air cemetery with tombs and mausolea.

Pope Pius XI, born Ambrogio Damiano Achille Ratti, was the Bishop of Rome and supreme pontiff of the Catholic Church from 6 February 1922 to 10 February 1939. He also became the first sovereign of the Vatican City State upon its creation as an independent state on 11 February 1929. He remained head of the Catholic Church until his death in February 1939. His papal motto was "Pax Christi in Regno Christi", translated as "The Peace of Christ in the Reign of Christ".