The Hölloch (English: Hellhole)[2] is a 200.4 km (124.5 mi) long cave in the municipality Muotathal in Switzerland.

[3] The cave was first scientifically explored in 1875 by a group led by Alois Ulrich.

[2] Later expeditions in the 1950s by Alfred Bögli, one of the pioneers of speleology, managed to explore a large part of the cave.

[2] Despite this, exploration of the Hölloch continued, and in 1976 it had been mapped to approximately 130 km (81 mi) long.

[2] 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.