It is formed where a bed of Triassic gypsum, emplaced within impervious Carboniferous strata, has been dissolved by groundwater.

[1] With a length of 300 and a width of 20 m it is the largest underground lake in Europe.

Prior to 1946 the water-level was much higher, but an earthquake with a force of 5.6 on the Richter-scale opened additional fissures in the cave on January 25, 1946 made it more readily navigable.

The visit is about half an hour long, and is held in English, French, German and Italian.

Access to the lake was closed from 2000 to June 2003, whilst the stability of the site was improved by the addition of more than 5000 bolts driven into the ceiling.