One of the activities included on the Altitude Summer Camp is a visit to the underground lake of St-Léonard. This mysterious cavern was discovered in 1943 by Jean-Jacques Pittard and was opened to public in 1949. The underground lake is the largest of its kind in Europe with a length of approximately 300 meters and can be enjoyed on a boat trip lasting around 30 minutes. The Lac Souterrain is about an hours drive from Verbier.
Lac Souterrain de St-Léonard
M. Cédric Savioz
Rue du Lac
More info: http://www.lac-souterrain.com/