Eisriesenwelt, the ice giants

Eisriesenwelt means World of the Ice Giants, and at over 42 km it is the largest ice cave in the world. About 200,000 people a year go into it.

The cave runs into the Hochkogel mountain in the Tennengebirge, about 40 km south of Salzburg, in the largest karst plateau in the Salzburger Alps. The mountains formed in the late Tertiary, and the cave was cut by the Salzach river eroding passages into the limestone.

Here is the part that surprises people: only the first kilometre is icy. The remaining 41 km is bare limestone. The ice is a feature of the entrance zone, and it exists because of how the cave breathes. The entrance stands open all year, and in winter cold air blows in and freezes the snowmelt that has drained into the cave. In summer the flow reverses, and cold air pushing out from inside the mountain keeps the formations at the entrance from melting. The cave is, in effect, refrigerating itself, and the ice grows or retreats depending on how much water got in the previous season.

That first kilometre is what the tour covers, and it is entirely ice: sheets, falls and columns of it.

Getting there is part of the day. The cave mouth sits high on the rim of the plateau above Werfen, so reaching it means a climb from the valley, and the walk continues once you are inside. It is around freezing in there whatever the month outside is doing, so take a warm layer and footwear you can climb steps in. The cave is seasonal.