Lac de Cazaux-Sanguinet is one of the largest natural freshwater lakes in France, straddling the boundary between the Gironde and Landes departments on the southern edge of La Teste-de-Buch commune. The lake covers about 5,600 hectares and is fed entirely by groundwater percolating through the coastal pine forest — no rivers flow into it, which gives the water unusual clarity.
In summer the lake is popular for swimming, sailing, windsurfing, and kayaking. The water is shallow and warm near the southern shore, making it good for families. The surrounding pine forest, part of the vast Landes de Gascogne forest, comes right to the water's edge on most sides, giving the lake a wild, undeveloped feel despite its proximity to the Arcachon basin.
There are several campsites and beach areas around the lake, and a cycle path connects the lake to La Teste-de-Buch and onwards along the coast. The Landes Canal, which links the lake to the Bassin d'Arcachon via a series of water channels, is worth exploring by kayak if you have the time.