The Ordinskaya Cave (or "Orda Cave"), which lies beneath Russia's Ural Mountains, is the world's largest gypsum crystal cave and one of the world's longest underwater caverns, with many areas still unexplored.
Hidden in the depths of the Vietnamese jungle lies The Hang Son Doong, part of a network of over 150 caves. Surrounded by jungle and used in the Vietnam war as a hideout from American bombardments, the cave passage is so large that it could hold a block of 40-storey skyscrapers.
Freediver at the entrance to Mariner's Cave! This is another gem of Vava'u... a hidden underwater cave only accessible via a swim into the darkness, coming up through the surface of the mirror-like cave chamber.
Grotto at Hamilton Pool, water cascading from above into Texas' best secret swimming hole (just outside Austin) by mlhradio, via Flickr (Tried to go here, not secret, 40 min wait to get a parking spot. -CKC)