Tucked away in south eastern France, the Gouffre Berger Cave is another one of those 'gorgeous but will probably kill you' attractions. If its nickname, "The Killer Cave" doesn't have you convinced, perhaps the fact that it's 3,681 feet deep will. It literally takes between 15 and 30 hours to return to the surface from the bottom of the cave. Within the cave are truly wonderful lakes and waterfalls, as well as structurally incredible caverns.
But heavy rainfall, combined with the lengthy amount of time it takes to exit the cave make exploration a recipe for disaster. Water rises fast, and there's no time to make it out of a flood. Cavers at Goueffre Berger have lost their lives, including an entire group of five in the '90s. If you value your life, maybe just check out the pictures online.