In the middle of the Valais valley, just below some beautiful vineyards, lies the inconspicuous entrance to one of Europe’s most impressive underground sites: Lac Souterrain de Saint-Léonard. It is the largest subterranean lake in Europe, with a length of 300 meters and a width of 20 meters.
After entering and descending the 50 steps you find yourself on a jetty, with waters of the continent’s largest natural underground lake stretches out in front of you. Its total area is approximately 6,000 square meters, and the water stays at an average temperature of 11 degrees Celsius (52 degrees Fahrenheit). The water is so very still that every drop hitting the surface can be heard distinctly in the cave.
The underground lake is located in a bed of Triassic gypsum. It formed over millions of years as the rock was slowly dissolved by groundwater. On the guided boat tour bats or the settled trout can be observed, and so can the bare gypsum, slate, and marble walls touching the edge of the water.
Know Before You Go
Sadly the lake isn't accessible for wheelchairs due to the mentioned stairs.
Like (almost) everything in Switzerland, the lake can easily be visited using public transport.