The Hölloch () is a long cave in the municipality Muotathal in Switzerland. In addition to being the second longest cave in Europe, it is also notable for having a depth of . The Hölloch is an example of a karst cave system. Exploration The cave was first scientifically explored in 1875 by a group led by Alois Ulrich. Later expeditions in the 1950s by Alfred Bögli, one of the pioneers of speleology, managed to explore a large part of the cave. The explored length of the cave increased from in 1952 to by 1968. It was the first cave in the world where the explored length reached 100 km, and until the linkage of the Flint Ridge Cave System to the Mammoth Cave System in 1972, it was believed to be the largest cave system in the world. Despite this, exploration of the Hölloch continued, and in 1976 it had been mapped to approximately long. Tourism A portion of the cave near the entrance is open to visitors, but the remainder is restricted to qualified cavers owing to its enormous size. See also List of longest caves References External links Hoelloch Cave Research Association Caves of Switzerland