The lake is located within a gypsum mountain, which is a rare geological occurrence for large subterranean water bodies.
Following a major earthquake in 1946, the lake level dropped significantly, forcing a temporary halt to tourism operations.
The cavern is approximately 260 meters long and 20 meters wide, with a total surface area of roughly 6,000 square meters.
Divers have explored the depths of the lake, mapping underwater passages that remain inaccessible to the public boat tours.
The water is remarkably clear, often allowing visibility of the rocky bottom throughout the tour route.
The Saint-Léonard underground lake is Europe's largest navigable subterranean lake. Located in the Valais region of Switzerland, the cave system spans approximately 300 meters in length and 20 meters in width. The water remains at a constant temperature of 11°C, while the air inside the cave stays around 15°C year-round. Boat tours navigate the cavern, allowing visitors to observe the geological formations and the clarity of the water. The site was formed within a gypsum rock mountain and is subject to natural water level fluctuations. Access is provided via a guided boat trip that lasts approximately 30 minutes. The cave was opened to the public for tourism in 1949 after being fully explored in 1943. It remains a significant geological feature of the Canton of Valais.
The center of the lake provides the best perspective to capture the vastness of the cavern and the reflections on the water.
Bring a light jacket or sweater, as the cave temperature is significantly lower than the outside summer heat.
Wear comfortable, non-slip footwear to safely navigate the stone pathways leading to the dock.
Arrive early in the day during peak summer months to secure a place on a boat as tours can reach capacity.
Do not attempt to touch the water or the cave walls during the boat tour to help preserve the fragile ecosystem.
Open seasonally from spring through late autumn; typically closed during the winter months.
Maintain a quiet tone inside the cavern to preserve the acoustic environment and respect the guide's presentation.
