Lake Of St-Leonard

The Lac Souterrain de Saint-Léonard, in the Valais, is a lake inside a mountain, and it is commonly described as the largest navigable underground lake in Europe. It lies beneath the hillside near the village of Saint-Léonard, in the Rhône valley between Sierre and Sion, and it can be visited on short guided boat trips. The lake is about 300 metres long, and its water is clear and still, lying in a large cavern with a vaulted ceiling. It was discovered in 1943, when a group of local people exploring the area found the entrance to the cavern, and it was later opened to the public as a natural attraction.

The lake lies in a cave formed within the rock of the hillside, and the water is part of the groundwater system. The rocks around it are layered, and the cavern has been shaped by the slow dissolving action of water. Geologists have studied the site as an example of how underground water can carve chambers, and the walls show the rock layers and mineral deposits, such as gypsum. The temperature inside stays steady all year, around fifteen degrees Celsius, so the cave feels cool in summer and mild in winter. The water level changes with the seasons and with rainfall, and it affects the tour, which is occasionally suspended if conditions require.

The visit begins at the entrance on the hillside, where visitors gather and a guide explains the rules and some background. A path leads into the mountain, and the group steps into a flat-bottomed boat that holds a number of passengers, pushed across the lake by the guide. The boat glides across the still water in the half-light, with lights picking out the rock walls and reflections on the surface, and the guide points out the shapes in the stone and tells the story of the discovery and exploration. The trip lasts about half an hour, and the sound of dripping water and the cool air give it an atmospheric quality. Guides speak several languages, and information is given in the main ones.

The site includes an area with a short presentation, using projection, that explains the geology and the formation of the cavern, and a small shop sells souvenirs. The experience is suitable for most visitors, including families with children, though the steps and boat boarding may be difficult for people with limited mobility, so ask in advance. Because the cave is damp and cool, a light jacket is advisable, and shoes with good grip are better than sandals.

The site is open in the main part of the year, from spring to autumn, with tours at set times, and booking is recommended, especially in summer and on weekends, since capacity is limited and the boats are small. Opening dates, prices and schedules vary, so check the official website. The lake can be combined with visits to Sierre, Sion and the vineyards nearby. It suits families, curious visitors and anyone who enjoys unusual natural sights. Allow about an hour in total.