The "Scavi" (excavations) are the archeological digs begun in the 1940's of early Christian tombs and the original St. Peter's Basilica constructed in A.D. 324. They are underneath the current basilica.

Tours can be booked for the Scavi via ...
~ Phone: 011-39-06-69885318
~ Fax: 011-39-06-69885518
~ email: scavi@fsp.va or uff.scavi@fabricsp.va
A website with a brief description of the tour: Reservations for the Vatican Scavi

Note that there are only a few tours offered per day, and fewer than 10 people allowed per tour. When requesting a reservation, be very specific about the date(s) you're interested in, and try to book as far in advance as possible.

Some people have mentioned that this tour is not for the claustrophobic!

Visitors to the Scavi must be at least 15 years old.

For a wonderful description of the Scavi and the story of St. Peter's tomb, I recommend the book, "When in Rome, a Journal of Life in Vatican City" by Robert J. Hutchinson. Here's a brief sample of his chapter called, "How the Vatican Lost St. Peter's Bones"..."These excavations were and still are one of the most astonishing finds in twentieth-century archeology, guaranteed to send a Raiders of the Lost Ark thrill racing up your spine."