Lechuguilla Cave is part of the Carlsbad Caverns Natural Park in New Mexico and is regarded as one of the most beautiful caves, with some of the most unique geography, in the entire world.
You can't visit.
Because of the delicacy of many of the formations, the cave is only open to scientists and the explorers who are still figuring out what all is down there. Nobody else is allowed in. Or, rather, nobody else but David Attenborough.
This video from the Planet Earth TV series takes you down into Lechuguilla for some amazing sights and fascinating commentary on the chemistry and biology that make this cave so strange and lovely. Even more impressive, nobody knew it was there until 1986.
Psst, Nova has a whole page on Lechiguilla, if you want to read more.