Saint-Léonard underground lake is located at Saint Léonard in the canton of Valais. It is formed where a bed of Triassic gypsum has been dissolved by water. 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. Its water is constantly at 11C. There is a population of rainbow trout that is being used to monitor the water quality and there are monthly concerts on the lake.
