Cave of the Crystals or Giant Crystal Cave is a cave connected to the Naica Mine 300 metres ft) below the surface, in Naica, Chihuahua, Mexico. The main chamber contains giant selenite crystals (gypsum, some of the largest natural crystals ever found.
The Chandelier Ballroom in the underground Lechuguilla Cave of Mexico, with its 6 meter long crystals formed naturally from chemical reactions in the complete absence of sunlight... featured on BBC's "Planet Earth": http://www.youtube.com/watch?v=ViuWH7LgoNo
Exploring the Longest Underwater Cave in Russia. Ordinskaya or the Orda Cave is the longest underwater cave in Russia. Accoridng to the Orda Cave Awareness Project it is the first and only underwater gypsum excursion cave in the world.