1. Mammoth Cave, USA
If you have certain expectations from somewhere called ‘Mammoth Cave’, that is entirely understandable. Mammoth Cave in Kentucky is the world’s longest known cave system, an incredible 420 miles of subterranean wonder. That’s twice as long as the next longest, by the way, although it isn’t unusual for the US to go all out on such things. Madly, the cave is home to some vast theatres and avenues, and tours have been known to last more than six hours.