The catacombs. I'll explain these to you briefly. All they are, or what they are, depending on how you look at them, are two tombs of people inside a mountain wall. It's really small and stuff, but that's really all it is, two tombs. This is a picture of the outside, they're on the right there.
A view from the catacombes above St Peter Cemetery to the graves around the church.
There is an entrance fee for visiting.
This cemetary is an amazing site and not filled with as much sorrow as other cemetaries around Europe (like in Prague's jewish cemetary). The catacombs are magnificent.