The Saint-Léonard underground lake
The Saint-Léonard underground lake (Lac souterrain de Saint-Léonard in french), near Sion, is the largest underground and navigable lake in Europe. It was formed in the 1940s following an earthquake that caused cracks and a lowering of the water level that filled the cave. The entrance to the cave is through the small refreshment bar.
The cave is located a few dozen meters under the vines and receives its water by rustle of rainwater and emerges through cracks. The cave was formed by the dissolution of Gypsum, a rock that dissolves over time in water. Other rocks such as marble provide waterproofing. The dimensions of the cave are 300 meters long and 10 meters high. The temperature inside the cave is about 15 degrees. A lot of information in three languages is available on the official website.
It is possible to park below the entrance of the lake at 5-10 minutes walk.
Visits to the lake date back to the 1950s just after the lake appeared. Access to the cave is via a refreshment bar where you can have a snack. The visits last about 45 minutes and take place approximately every hour with a multilingual guide in boats of about 20 people. The lighting of the cave allows to discover its structure and the different layers of rock that make it up. We are surprised by the calm and transparency of the lake’s waters. It is about 2 metres deep and contains fish that have been introduced to monitor the cleanliness of the water.
Once a month concerts take place from a boat that you can see when you visit the cave which has excellent acoustics. Aperitifs can also be taken on reservation.