Orda Cave (Russia)
Orda Cave is the world's longest underwater gypsum cave. Its crystal clear water provides incredible views of its craggy paths and expansive chambers and makes the cave as dangerous as it is beautiful. The brittle gypsum that makes Orda so beautiful also makes it perilous, as chunks the size of cars frequently shear from the ceiling. Only experienced adventurers should brave its dangerously low temperatures and labyrinthine network of passageways.