Eisriesenwelt
The Eisriesenwelt (German for 'World of the Ice Giants') is a natural limestone and ice cave located in Werfen, Austria, about 40 km south of Salzburg. The cave is inside the Hochkogel mountain in the Tennengebirge section of the Alps. It is the largest ice cave in the world, extending more than 42 km and visited by about 200,000 tourists every year.
Cave data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).