Catacombs under Paris
If in a life you get a chance to meet a ‘cataphile’ (a catacomb addict), ask him to take you down there. It’s definitely a unique experience walking in this underground network of tunnels & galleries. Better go with a map and an experienced guy as there’s no way you find your way out alone. You’ll indeed meet people down there from time to time and enjoy the overall friendliness of these underground life addicts. Actually it’s true : Once you’ve been there and despite the dirt, the risk of encountering police or hit your head on the ceiling, there’s a strong appeal to get back there again.