Recently I had the privilege of visiting Narni Underground. If you liked Dan Brown's books, come meet a real person who is making these kind of discoveries, not fictional ones!
Roberto Nini and his friends were just kids when in the 70s' they discovered this hidden jewel: a base of operations for the Catholic Inquisition. What I also found remarkable was that it is recent and unique (there are very few testimonies to the operations of the Inquisition that have not been destroyed or hidden).
You also see a beautiful Dominican chapel on this visit complete with frescoes and yes, even bones! I look forward to going back because I have a feeling that more discoveries will follow.
Bring walking shoes. There are some stairs. No photos allowed.