Since graveyards closed more and more during the Revolution, the Parisians needed somewhere to put all those bones, thus they started mass quarry-graves. At the beginning of the 19th century, there were bones of more than six millions citizens in the catacombs, including the famed head of Marie Antionette! This is definitely a cool sight to check out, but beware if you're claustrophobic... the walk down into the tunnels can be very freaky. Spooky fun!