Almost midway between Sion and Sierre is the small village of St-Léonard with, as its sole draw, one of the largest underground lakes in Europe (March–Oct daily 9am–5pm; Fr.6). Regular buses between Sion and Sierre drop off either in the village or at the car park beneath the ticket office, which can get crowded with day-tripping families. Tours of the lake, by large rowing boats with everyone crowded in side by side, take about forty minutes – and it’s cold down there, so bring a jumper. The cave entrance is just a gap in the mountainside, but as you launch off onto the inky water, the illuminated, other-worldly cavern stretching out ahead of you is very impressive. The guides, who do the rowing, have a nice line in multilingual patter, and regale you on the voyage with all kinds of stories and details about the lake and its geology.