The underground lake of St-Léonard

11 June 2025

Humans are not the only ones who build tunnels. Nature, too, has a reputation in this field, for example, in Saint-Léonard (Canton of Valais), Vallorbe (Canton of Vaud), and Baar (Canton of Zug).

St Léonard is even home to the largest navigable underground lake in Europe, the Lac Souterrain de St Léonard.

This 300-metre-long, 20 to 30-metre-wide, and several-metre-deep lake lies around 60 metres below the mountains and has been formed over thousands of years. Water from springs, meltwater from snow and ice, and rainwater have displaced the soft rock gypsum. The other types of rock (marble, slate, and anhydrite) now form the backdrop.

Initially, there were many separate spaces, but gradually, the underground lake we know today was formed. Residents and experts had been aware of this lake for a long time, but as the water reached right up to the roof of the cave, it was not possible to conduct any investigations for a considerable period.

Their research boat is now a museum object.

Two speleologists first investigated and documented the lake in 1943. However, the breakthrough only came with the 1946 earthquake, which caused cracks in the walls, allowing the water to drain to its current level.

The stage for the artists

With a glass of wine from Lac Souterrain St. Léonard

The lake is navigable by boat, and there are even musical performances; the wine also thrives thanks to the constant temperature of 15 degrees Celsius.

Even the first stalactites began to form after 1946. Until then, there was no air and no oxygen, only water. It also shows how much time stalactites need to equal Vallorbe or Baar.

What applies to the salt in Bex also applies in St Léonard: nature knows no time or takes its time.

St. Léonard