The Vatican Necropolis lies under the Vatican City, at depths varying between 5–12 meters below Saint Peters Basilica. The Vatican sponsored archeological excavations under Saint Peters in the years 1940–1949 which revealed parts of a 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 and 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, 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. One of these streets, the Via Cornelia, ran north along the Vatican hill, at the top of the circus that Caligula built, an Egyptian obelisk had been placed. The obelisk had been there since ancient times, in 1586 it was moved from its place by Domenico Fontana on the orders of Pope Sixtus V when St. Peters Square was added. The original location, just before the current excavation Office of the Fabbrica di San Pietro, is marked by a plaque in the ground, according to tradition, the Apostle Peter was martyred in the year 64 or 67 during the reign of Emperor Nero. Peter is said to be buried in the necropolis because of its proximity to the Circus of Nero where he was martyred, after the Edict of Milan the Emperor Constantine began construction of the first St. Peters Church, also known as Old St. Peters Basilica. At this time, the Roman necropolis was still in use and this is known because a coin was found inside an urn dating from 318 AD. During this time, the necropolis was protected by law and was untouchable, however, Emperor Constantine I decided to build a basilica, which would be located just above the supposed grave of the Apostle Peter. To obtain the amount of flat area for the planned construction, Emperor Constantine I excavated part of the necropolis of the Vatican hill. This caused the necropolis to be filled with soil and building debris, with the exception of St. Peters tomb, 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, a series of mausoleums were unearthed. The mausoleums were initially labeled with the Greek alphabet letters Φ, Χ and Ψ, 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 part of the foundation for the canopy of Gian Lorenzo Bernini was created. First, the A mausoleum was built, in later years, in rapid succession, the mausoleums B, C, D and E were built next to each other. The Mausoleum G is very likely from the time as Mausoleum B
Christ with the attributes of Sol Invictus. Taken from a mosaic from the necropolis under St. Peter's Basilica in Rome.
Cross-sectional profile beneath St. Peter's Basilica
Map and location of the cemetery in relation to St. Peter
Mausoleums in the Vatican Necropolis with temporal classification of buildings