Explore the ancient caves of Hang Son Doong, Vietnam. Hang Son Doong is one of the largest caves in the world and features a fast-flowing underground river. For spelunkers, exploring Hang Son Doong is the equivalent of scaling Mount Everest.
Ice Cave Near The Mutnovsky Volcano, Russia - Ice caves like these form in the glaciers surrounding the Mutnovsky Volcano in Russia. Some of them are formed by vents that release volcanic heat and gases called fumaroles. (photo by Florian Wizorek)
Cave of the Crystals or Giant Crystal Cave is a cave connected to the Naica Mine 300 metres below the surface in Naica, Chihuahua, Mexico. The main chamber contains giant selenite crystals, some of the largest natural crystals ever found.