Lac Souterrain |
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15 September 2025 - Saint-Léonard
Today I saw Europe's largest underground lake, Lac Souterrain in Valais, Switzerland. It was very long and thin as a result of its unique geology. One side of the lake is Carboniferous schist. The other side is Triassic marble. In between is a 20m layer of gypsum that is not nearly as resistant to water as the other two. Erosion of this gypsum layer is what created room for the 300m lake and the 1500m of caverns beyond it blocked off by a cave in at some point before humans discovered the lake. Amazingly this was only in the 1940's. While not as big as the Lost Sea in the karst of Tennessee, Lac Souterrain is still impressive.
