The first excavations of the Necropolis occurred from 1940–1949 during the pontificate of Pius XII. The purpose of these excavations was to locate the grave of St. Peter, which for centuries had been assumed to be beneath St. Peter’s Basilica.
The Vatican Necropolis lies under the Vatican City, at depths varying between 5–12 meters below Saint Peter’s Basilica.
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.
The Vatican Necropolis is not to be confused with the Vatican grottoes, the latter of which resulted from the construction of St. Peter’s Church and is located on the ground level of the old Constantinian basilica.
The Vatican necropolis was originally a burial ground built on the southern slope of the Vatican Hill, adjacent to a circus built by Emperor Caligula. In accordance with the Roman law, it was forbidden to bury the dead within the city walls. For this reason, burial grounds sprang up along the roads outside of the city cemeteries.
A series of mausoleums were unearthed. The mausoleums were initially labeled with the Greek alphabet letters; later, Latin letters were used.
The Mausoleum M had already been described in 1574, and Mausoleum O was discovered when it was unearthed during the construction of the foundation for the statue of Pope Pius VI. Mausoleums R and S were discovered when the southern part of the foundation for the canopy of Gian Lorenzo Bernini was created. Some tombs have undergone restorations, such as the ten-month project involving the Valerii Mausoleum.
The field named P (Peter Campus) is the small area in which the suspected grave of the Apostle Peter is located. Peter was, according to tradition, after his martyrdom in the Circus Nero, buried here. Some 100 years after the death of Peter, a shrine was erected over his grave. This shrine is adjacent to the so-called Red Wall. Immediately adjacent to the suspected Peter grave, some other tombs were found. The arrangement of the graves suggests that the place of Peter’s tomb was the site of some early veneration. The shrine, also called the “Trophy of Gaius”, is named for the theologian Gaius of Rome who lived in Rome during the time of Pope Zephyrinus (198-217 CE).