Visit the pre-Constantine necropolis under St. Peter's for a fascinating glimpse of the underpinnings of the great basilica, which was built over the cemetery where archaeologists believe they have found St. Peter's tomb. Apply in advance by sending a fax or e-mail (scavi@fsp.va) with the name of each visitor, language spoken, possible days for the visit, and a local phone number. Reservations will be confirmed a few days in advance. No children under 15 are admitted. Tickets are sometimes available for same-day tours; apply in person at the Ufficio Scavi (Excavations Office), on the right beyond the Arco delle Campane (Arch of the Bells) entrance to the Vatican, which is left of the basilica. Tell the Swiss Guard you want the Ufficio Scavi, and he will let you by. COST: EUR9.