St-Leonard Underground Lake

The lake fills a cavity in gypsum and marble about three hundred metres long and up to twenty wide, and it is billed as the largest navigable underground lake in Europe - a claim that rests on the boat traffic rather than raw volume. It was formed by water dissolving gypsum, and its level and shape have changed within recorded history: the 1946 Valais earthquake shifted the rock and drained part of it, and the modern visit dates from the reopening afterwards. Visits are by rowing boat with a guide, about half an hour on the water, with the ceiling close overhead and the light arranged to show the rock rather than to flood it. Water temperature stays around eleven degrees and the air is cool and damp.

What you actually see

• A 300-metre underground lake - crossed by rowing boat with a guide.
• A ceiling close overhead - the low rock roof through most of the route.
• Gypsum and marble - the soluble rock the cavity formed in.
• The 1946 earthquake's mark - the event that changed the lake.

When to go

Mid-March to end of October. An hour.

Honest expectations

Closed all winter. Visits are guided boat trips only, in French with other languages by arrangement; there is no walking route. It is cool and damp - bring a layer. Queues form at midday in summer and the boats are small.

Nearby

Sion and its castles are west; Sierre and Val d'Anniviers are east.