Les Catacombes de Paris
You exit the Metro at Denfert-Rochereau, cross the street, enter the little building and pay the admission price, then descend 130-something steps down a steep spiral staircase. From there, you walk and walk through dimly lit tunnels until you come to a doorway with a large sign admonishing you to touch nothing and to not smoke. Through the doorway begins a walk through the largest cemetery imaginable. You walk past thousands and thousands of human bones, stacked neatly and artfully in sections marked by signs indicating which cemetery they come from. It's an interesting and creepy tour, and well worth your time if you're in Paris.