Hölloch

The Hölloch (Hellhole) is a 200.4 km long cave in the municipality Muotathal in Switzerland. In addition to being the second longest cave in Europe and the 11th longest of the world, it is also notable for having a depth of 1033 m. 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 25 km in 1952 to 100 km by 1968. It was the first cave in the world where the explored length reached 100 km. Until the linkage of the Flint Ridge Cave System to the Mammoth Cave System was discovered 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 130 km long.

Tourism
A portion of the cave near the entrance is open to visitors, but the remainder is restricted to qualified cavers only owing to its enormous size.