The Hölloch is a 200.4 kilometres (124.5 mi) long cave in the Muotathal municipality in Switzerland. It is also notable for having a depth of 938.6 metres (3,079 ft) 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 25 kilometers (16 mi) in 1952 to 100 kilometres (62 mi) in 1968 (it was the first cave in the world where the explored length reached 100 km).