'Roma Sacra - The Vatican Necropolis'.

Here is the official information we were given on the Scavi tour:

The rules of the tour are fairly strict. No cameras. No large bags. Proper dress.only persons over 15 years old are admitted.
The tour is actually finished when you enter the Clementine Chapel in the grottoes. Your guide might allow you to then visit the grottoes (tomb of the popes).
As we enter through the Archway of the Bells, we come to the scavi office, the entrance to the excavations beneath the basilica. Scavi tours are not large affairs, and as we go down the stairways and enter the excavations themselves, you can see why. The passageways are narrow and slightly musty, even dampish. As we make our way through the dark corridors that were once streets and alleys in the Vatican Hill necropolis, our guide points out the elaborate pagan funerary monuments as well as Christian tombs. There, after about a twenty-minute walk, is what can be made out of the Tropaion. And after that, reinterred in the graffiti-marked wall I mentioned before, are what the guide tells us are the mortal remains of Peter the apostle. Leaving through the gilded baroque splendor of the Clementine Chapel, you can't help but think that what we've just seen and touched and smelled is about as close to the apostolic roots of the Catholic Church as it's possible to get.
THE SCAVI (Tomb of Saint Peter):

One of the most interesting tours in Rome is a visit to the excavations in the Necropolis (City of the Dead) underneath Saint Peter's Basilica. It was there that Saint Peter was buried, after having been crucified in the Circus of Nero that sat nearby (where the Vatican Gardens are located). This 60 minute tour takes you in a specific language group through several excavated streets of the Necropolis and ends at the grave of Saint Peter. Tickets normally are reserved in advance as space is limited.

You can apply for the Scavi Tour in person by visiting the Office of the Scavi (go through the Vatican gate to the left of the Entrance to St. Peter's) or you can fax at 011.3906.698.85518. DO NOT TELEPHONE THIS OFFICE FROM THE UNITED STATES! If you fax, be sure to include the full names of all people, the language in which you request the tour to be conducted, and the name and phone number of your hotel in Rome. Failure to include any of this material, may cause your application to be turned down! You can then visit in person or have your hotel call after you arrive to see if your reservation was accepted. The Scavi also has a new email address at scavi@fsp.va which seems to be working.

Please note that it is VERY DIFFICULT now to get tickets to the Scavi Tour, and the Scavi Office is NOT always the most helpful. With the large amount of visitors to Rome in the past year and the increased publicity about this tour, it is harder and harder to get reservations. Please try, but don't be too disappointed if you are not successful. The earlier you try, the better, as long as you know for sure the exact days and times you will be available. They don't take it kindly if you ask for a different time after receiving a confirmed reservation.