3. Lechuguilla Cave
If you have ever wondered what speleotherms, gypsum chandeliers and hydromagnesite balloons look like, the answer is you will never be able to find out. All these wonders can be found, supposedly in the Lechiguilla Cave in New Mexico. This closely guarded cave is totally off limits to all and sundry. You can get a permit to delve down them but you would have to be an extremely good caver, and even then the chances of you getting permission is very slim. It’s supposed to be incredible but certainly one of the places you will never visit.