The Vatican sponsored archeological 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 underground Catacombs of Rome, but an open air cemetery with tombs and mausolea.
Read more: Sacroprofano Splendor Of The Vatican Grottoes.