The “Lac Souterrain de St-Léonard” owes its existence to the superposition of phenomena specific to geology, plate tectonics, hydrology and its geographical location. Its formation took place in several stages and is still continuing today. It all begins with the appearance and emplacement of the three rocks surrounding the lake. The north face is formed of black shale or “coal shale”, a rock of Carboniferous age (more than 300 million years old), while the south face is formed of marble, “the stone of St-Leonard”. These two rocks, totally watertight, will allow the lateral retention of water inside the cave. Placed in the middle of these two rocks, we find gypsum or “plaster stone”, a sedimentary rock, porous and soluble in water over time. This gypsum, which appeared 220 million years ago, when Valais was just a vast ocean, would have been pushed between the marble bar and the shale bar during the formation of the Alps. While the appearance and emplacement of these rocks took place over several hundred million years, the excavation of the cavity, by the action of water on the gypsum, is thought to date back only a few thousand years. One hypothesis even suggests that the erosive process began in the post-glacial period, less than 12,000 years ago. Runoff water seeps through the permeable gypsum, favoured by the numerous cracks caused by the frequent earthquakes in the region, and also by the geographical location of the lake, at the foot and on the side of the mountain. These waters descend along the slope and infiltrate underground through the gypsum, forming, by “dissolving action”, underground channels. Over time, these channels expand to form wells that bring and store the water deeper. These waters will cause a phenomenon of “desquamation”, forcing the waterlogged gypsum to swell in volume, causing it to detach and collapse towards the bottom of the cavity being formed. This erosive process allowed the formation of the “Lac Souterrain de St-Léonard”, whose dimensions are still expanding today.