Wikipedia
Hölloch
The Hölloch is a long cave in the Muotathal municipality in Switzerland. It is also notable for having a depth of and being the second longest cave in Europe. The initial exploration started in 1875 and was led by Alois Ulrich. A large part of the exploration of this cave was led by one of the pioneers of speleology, Alfred Bögli. The explored length of the cave increased from in 1952 to in 1968 (it was the first cave in the world where the explored length reached 100 km). Until 1970, it was thought to be the largest cave complex in the world, this title now held by Mammoth Cave.