Right before we did our night time tour of the Vatican Museums, we went to climb the dome of St. Peter's Basilica, as it's also in the Vatican City. Never did we imagine walking into mass when we arrived at the base of the dome. Seeing all of it and hearing the singing brought us to tears. It was a surreal and powerful experience. Just the building and all the art work were overwhelming. You can also reserve in advance for the scavi tours, to see the tombs and excavations underground. Because these are holy grounds, you have to dress appropriately or you will be turned away. Please have shoulders covered and no bottoms above the knees. This applies to all churches you visit, including the catacombs.