Austria: Eisriesenwelt cave
The Eisriesenwelt (World of the Ice Giants in German) is a natural limestone and ice cave located in Werfen, Austria, about 40 km south of Salzburg. It is the largest ice cave in the world, extending for more than 42 km.
Eisriesenwelt was formed by the Salzach 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. Since the entrance to the caves is open year-round, chilly winter winds blow into the cave and freeze the snow inside. In summer, a cold wind from inside the cave blows toward the entrance and prevents the formations from melting.
The cave is open from May 1 to October 26 every year. Temperatures inside the cave are usually below freezing, and warm clothing is recommended. Photography is not permitted once visitors are inside the cave. The round-trip tour through the cave takes around one hour and 15 minutes.