La Verna Cave - The Largest Cave system in the world and can hold 10 Cathedrals the size of Paris Notre Dame

..and one can fly a hot air balloon inside also.................

Located about 45 minutes outside of the city of Pau ( next to the French Pyrenees ), it can be visited almost all year.

Follow in the footsteps of the first explorers, accompanied by a qualified caving guide and ardent enthusiast of the La Pierre Saint-Martin cave system. Starting from La Verna, this visit is a good introduction to caving. The narrow passages - where crawling would be required - can be avoided, but the terrain is rough, without paths. The visit takes you 
upstream all the way to another big cave room: