I was fortunate to get tickets to the excavations under St Peter's - the Scavi - as only 120 visitors are allowed down each day, and tickets have to be applied for in advance. The tickets cost 10 euros but no under 12s are permitted. You will be allocated a set time to arrive and taken down in a group of 12 by a guide. No backpacks are permitted and must be left in the office, nor is photography permitted. Once in the excavations the atmosphere is dusty, hot and humid and with the narrow corridors it is not the most pleasant environment.
The guides are well informed and give detailed information as you are lead around the excavations. The excavations under St Peter's discovered a necropolis, a burial place - a city of the dead - with streets and various rooms that belonged to wealthy Roman families where their dead relatives were entombed. Some of the rooms are beautifully decorated, including one where the ceiling depicts Christ as a sun god. The necropolis was discovered by accident but the excavations were commissioned to discover if it was true that St Peter was buried beneath the altar of St Peter's as it was claimed. This is the centrepiece of the tour, a small cheaper grave that has a ancient graffiti wall that indicates that early Christians believed Peter was buried here. The tour guide will explain that bones were removed and later replaced in a niche in a plastic box, which you are able to see. You can see the marble of the earlier altar of Constantine's basilica that is built directly on top of this grave, and the current altar is directly above this. Whether or not this is grave of St Peter or not, it certainly seems that early Christians believed that Peter had reached Rome and that he was buried here.
The tour ends in a small elaborate chapel before you are lead down a corridor into the grottoes were previous popes are buried, close to John Paul II tomb. If you are fortunate to receive tickets for the excavations, it is an incredibly interesting tour and worth enduring the oppressive atmosphere in it.