Saint-Léonard underground lake, Underground lake in Saint-Léonard, Switzerland.

The Saint-Léonard underground lake lies inside the Châtelard hill, a 260 m long and 29 m wide body of water at a constant 11 °C. Its rock is gypsum, schist and marble, dissolved by seeping water; iron-rich clay seals the lake bed. Pale tan and grey rock masses descend into the dark water, and warm yellow light strikes the walls of a further cave section behind a low opening.

Known to locals long before, the cave was first explored in 1943 by Jean-Jacques Pittard, accompanied by Jacques Della Santa. A 1946 earthquake near Sierre lowered the water level, and two locals opened the cave to visitors in 1949. A two-tonne rock fall in 2000 closed it until 2003, after ceiling reinforcement costing over 2.5 million Swiss francs.

At the far end of the cave stands a statue of Saint Barbara, patron saint of geologists, watching over those who venture onto the lake. Three boats lie deliberately sunk on the lake bed to give visitors an extra point of interest.

The cave is entered at the foot of the vineyards in Saint-Léonard, between Sion and Sierre. The water stays at 11 °C, so bring a jacket. Visits run daily from 19 March to 1 November; three boats carry about 40 passengers each. The far end of the lake is blocked by fallen rock, and the dry chambers discovered in 1956 beyond it are closed to visitors.

Three boats lie deliberately sunk on the lake bed to give visitors something to look for below the surface. Farther inside the cave stands a statue of Saint Barbara, patron saint of geologists, watching over those who set out on the water.