Today in the Catholic Church calendar is the Optional Memorial of The Dedication of the Basilicas of Saints Peter and Paul, so I thought this might be a good time to look at the necropolis, or burials, beneath St. Peter’s Basilica in Rome. This site dates to ancient Roman Imperial times and was the likely location of the burial of St. Peter after his martyrdom by the Emperor Nero in the Circus of Caligula between 64-67 AD. Rather than a catacomb, this site held family mausoleums and other above-ground tombs. Constantine the Great chose the location for his basilica because of its proximity to St. Peter’s remains.
For more information on the necropolis and its history, read the account by a priest who was a guide for the Vatican tours of the Necropolis. Wikipedia also has a decent article with further references.
These photos and many more are included in this brief video.