This is one of the creepiest places I have ever been. We made a visit to the catacombs on this past trip to Paris. About 6 million bodies are in these tunnels. A fraction of the tunnels is open to the public. It is enough to get that creepy feeling. It was perfect for a October visit! I have visited the catacombs years ago and still holds my imagination.
9 Creepy Things You Should Know About Paris’ Underground City Of Death from Buzzfeed
The French site for the Catacombs http://www.catacombes.paris.fr/