10. Eisriesenwelt Cave
The Eisriesenwelt (German for “World of the Ice Giants”) is largest ice cave in the world, extending more than 42km (26 miles). The cave is inside the Hochkogel mountain in the Tennengebirge section of the Alps in Austria. Eisriesenwelt was formed by a river, which eroded passageways into the mountain. The ice formations in the cave were formed by thawing snow which drained into the cave and froze during winter.