The Catacombs in Paris is one of the spookiest sites in the city and certainly one of the creepiest places I have ever been! It is basically a super old city crypt that you can go down inside (for a fee) and see just piles and piles of human bones, including whole skeletons. Some of the bones are even arranged into unsettling little displays, like a heart and a cross. It took us over two hours waiting in line (in the heat too, wah!) before we could get in, so make sure to book tickets in advance if you can. Prepare to be thoroughly spooked!