Hölloch, Cave entrance in Oberstdorf, Germany.

Hölloch is a natural karst cave in the Mahdtal valley on the southern edge of Oberstdorf, Bavaria, close to the Austrian border. Its surveyed passages run 12,900 metres in total, extend 1,823 metres horizontally and span a height difference of 452 metres. Entry is through a 76.6-metre-deep entrance shaft leading to the main passage.

Hermann Paul from Innerschwende and chaplain Xaver Lutz from Riezlern made the first descent of the 76.6-metre entrance shaft and the main passage in 1906, carrying out its first survey. In October 2005 a second entrance, the Plattenalpschacht, was opened to reach newer cave sections more easily; it is locked and not used for tourism.

The name Hölloch means "Hell Hole", and the cave has drawn filmmakers for decades: Gerhard Baur's 1977 Eine Seilfahrt in die Unterwelt, his 1994 Mit dem Wasser in die Tiefe and 2011 Hölloch – Geschichten aus der Unterwelt, plus Jürgen Schafroth's Highway to Hell and a 2019 Arte documentary. The Sonthofen cave club published a centenary volume on Mahdtal research in 2006.

The cave is an active water cave: meltwater or rain can enter within a short time and cut off the return route, so a stable weather situation is essential. The second entrance, the Plattenalpschacht opened in October 2005, is locked and not usable for tourism. Access from the Austrian side is possible through the Kleinwalsertal. Only trained, experienced cavers should enter.

The surveyed length is 12,900 metres, with a vertical range of 452 metres and a horizontal extent of 1,823 metres. The entrance shaft drops 76.6 metres. Since October 2005 a second entrance, the Plattenalpschacht, has opened the newer passages to research, but it is locked and closed to visitors.