Lac Souterrain St-Léonard

The Underground Lake of Saint Léonard

Useful Information

Location: Rue du Lac 21, 1958 Saint-Léonard.
6 km east northeast of Sion on the Sion-Sierre road.
(46.256446, 7.425778)
Open: Mid-MAR to JUN daily 10-17.
JUL to AUG daily 9-17:30.
SEP to 04-NOV daily 10-17.
Online booking required. ExplainPost-pandemic Aftermath
Fee: Adult CHF 12, Children (5-16) CHF 7, Children (0-4) free.
Groups (12+): Adults CHF 10, Children (5-16) CHF 7.
Classification: Speleologygypsum cave
Light: LightIncandescent Electric Light System
Dimension: L=300 m (cave), L=260 m (lake), W=29 m, Ar=6000 m², A=509 m asl, T=15 °C (air), T=11 °C (water).
V=100,000/a [2019].
Guided tours: D=40 min, completely by boat. English Deutsch - German Français - French
Photography: without flash allowed
Accessibility: no
Bibliography: André-H. Grobert (1987): The subterranean lake of Saint-Léonard (Valais-Swiss), 12 pp, 2 colour photos, survey, SB.
Address: Lac Souterrain de Saint-Léonard, Case postale 75, 1958 St-Léonard, Tel: +41-27-203-22-66. E-mail:
As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then.
Please check rates and details directly with the companies in question if you need more recent info.


APR-1943 first explored by members of the Société Suisse de Spéléologie.
25-JAN-1946 an earthquake creates fractures in the rock which results in loss of water.
1949 developed and opened to the public.
1950 the cave is consecrated to the Notre-Dame des Gouffres by a local priest. A sculpture is placed on the shore of the lake.
2000 cave closed for renovation.
16-JUN-2003 reopening.


At St. Léonard, on the main road between Sion and Sierre, on the right bank of the Rhône, 5 km from the capital of the Canton of Valais, is the largest underground lake in Europe. The cave is formed in gypsum, about 260 m in length and 15 m wide. The cave was discovered in 1943 and commercialised in 1949. Here, close to the entrance of the lake, visitors can enjoy a moment of agreeable relaxation with a large car park and a refreshment room set in a pleasant, quiet and cool garden. A boat trip on the wonderful clear water of the lake gives one an opportunity to admire the rich, harmonious and varied subterranean geology.

Text by Tony Oldham (JUN-2001). With kind permission.

The cavern with its lake was known for a very long time, at least the entrance and the shore of the lake. It is located in the middle of the vineyards and the vine-growers used to cool their bottles of wine in the cold water. But the locals did not enter the cave, and foreigners did not visit this secluded spot. The reasons is simple, they had no reason for entering. And probably there was a little superstitious fear. In the centuries before the cave was explored, several legends about the cave developed.

A young man was trapping around the cave entrance, and he discovered leaves of stone in the cave entrance and picked one up. To his surprise a heavy wind arose out of the cave, across the lake. When the wind calmed down again, the leaf had become two golden coins. But whenever other people tried to do the same, it never happened again.

Young maids, visiting the lake at midnight of December 24st, will see the reflection of their future husband in the dark waters of the lake.

The cave is located in a layer of anhydrite (gypsum) which can be found along the Rhone valley around St-Léonard. It reaches the surface for three kilometers, between St-Léonard and Granges. It was quarried sometimes, but this is not profitable, as the gypsum is very impure.

The first exploration, with the discovery of the huge lake, happened in 1943. Jean-Jacques Pittard and Jean Della Santa used an inflatable boat to enter the cave. When they landed on the shore at the far end, they had visited the whole cave and returned. The two young men from St-Léonard intended to show other people their discovery, but the water-level was much higher than today, in other words the ceiling was lower, which made boating much more difficult than today. This changed on 25-JAN-1946, an earthquake with 5.6 on the Richter-scale opened additional fissures, through which water from the cave flowed towards the water table of the Rhone plain. This made the lake more readily navigable, and it became possible to open the lake to the public in 1949. Since then there were numerous improvements made and today four boats of different sizes are used and even events are possible on the boats. At the beginning there was no light, the visitors were equipped with handheld lamps.

The cave has a small museum at the entrance which is actually called a didactic area. It's an interesting place to spend some time while waiting for the start of your tour. The site also has several musical events every year, which use the exceptional acoustic qualities. The number of places is very limited. Another event at the cave is the BAT Race, an underground race with SUP or stand up paddling boards. It is said to be part-athletic competition, part-pure novelty event.

The anhydrite was originally deposited in an aride climate during the Permian (220 Ma) in a cove of the Tethys sea. During the formation of the Alps, the layers of sediments were moved and displaced in so called nappes. The anhydrite of St-Léonard belongs to the Pennine nappe. After its dislocation it is now tilted by 90° and stands upright, enclosed by schist, Carboniferous coal shale, to the north and slightly metamorphic limestone to the south. The coal shale actually has nothing to do with coal, it's named after the black colour. The limestone is locally called marble and also “the stone of St-Leonard” as it was quarried for numerous purposes. The formation of the cave is much younger, it started 12,000 years ago at the end of the last ice age. Caves in anhydrite form very fast, the gypsum is well soluble in water. The melting water of the glaciers entered the gypsum along cracks, a result of frequent earthquakes, and the impermeable shale and marble prevented it from going astray. The cavern actually has the maximum size, while the ceiling and floor are anhydrite, the parallel walls of schist to the north and marble to the south are insoluble and prevented further growth.

Today the cavern still grows by small amounts of gypsum falling from the ceiling and dissolving in the water. But in winter 2000 a two ton rock fell from the ceiling into the lake, it was fortunate this happened while the cave was closed. Actually such an event is not unexpected in a gypsum cave, but the authorities closed the cave immediately for security reasons. The cave was closed for around 2.5 years for extensive renovation work. The cave ceiling was secured by inserting several thousand rock anchors which cost 2.5 Million CHF. The cave was reopened in 2003.

At the moment there is no real exchange of water. Connections to the surface seem to be sealed by clay, and only a little amount of water leaves the cave at the entrance towards the water table of the Rhone plain a few meters below. To keep the water at the same level some water is pumped into the cave.

The cave has replaced the normal ticket sale with online booking, one of the Corona measurements, actually. There are no tickets sold on location anymore, but late minute online purchases are allowed, so you can buy the tickets actually 15 minutes before the tour on the parking lot with your smartphone. Prebooking is much recommended, as the tours are often sold out during season. Even more important is prebooking out of season, as tours might be skipped if there are not enough visitors. Check in 15 minutes before the start of the tour.