Paris Catacombs
Not really open to the public, but below Paris are miles of man made Catacombs that were made by the Romans and later used by the Nazis and French Resistance. While there you can experience:
1. A lot of interesting Graffiti art
2. Nazi underground construction added to the Catacombs
3. Empire of the dead - a place where 6,000,000 people were buried (local cemetery)