If you get lucky on the Scavi tour (unlikely if you are only in Rome for a single port day) you will receivd an e-mail direct from the Vatican Scavi office telling you to report to the Swiss Guard at a gate located on the left side of St Peters. You will than have to show a copy of the e-mail and will be escorted to the small Scavi office. From there, you will be put in a group of 10 -12 and be given an underground tour that lasts a little over an hour. If you get claustrophobia its not a good tour (smile). The excavations are directly under St Peters Bascilica. I am glad we did this tour a few years ago, but I would not consider it a priority if this is your first visit to Rome. There are too many other things in Rome that would be much higher on my list.