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Arcachon Bay, in French the Bassin d'Arcachon (the locals just call "le Bassin"), is a bay of the Atlantic Ocean on the southwest sea shore of France, situated as a landmark between the Côte d'Argent and the Côte des Landes, in the region of Aquitaine. The bay covers an area of 150 km² at high tide and 40 km² at low tide. Some of its geological features are natural preservation areas.

The general shape of the Bassin d'Arcachon is that of an equilateral triangle pointing north, the southwest corner of which is opened and the access from the sea, between Cape Ferret and the town of Arcachon (more specifically, its suburb Pyla-sur-Mer), through a 3 km narrow channel (Les Passes). On the north shore is the town of Arès, then Andernos-les-Bains on the northeast. Just south of the entrance is The Great Dune of Pyla. Nearly in the middle of the bay is a very particular island: L'île aux Oiseaux (birds island).

It is perhaps due to the Eyre river that runs water from the Landes forest and has its mouth (Delta de l'Eyre) in its southeast corner, that the Bassin still has a link to the sea, which would otherwise be obstructed by the sandbanks brought about by the tides.

For long ago, similar areas became lakes (called in French "lacs" or "étangs") nowadays filled with fresh water; from north (the Gironde mouth) to south (the Adour river mouth), are the Lac d'Hourtin-Carcans, the Lac de Lacanau, the Étang de Cazaux et de Sanguinet, the Étang de Biscarrosse et de Parentis, the Étang d'Aureilhan, the Étang de Léon, the Étang de Soustons, the Étang Hardy, the Étang Blanc and the Étang de Garros. Arcachon Bay is the last water area that remains wide opened to the ocean.

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Coordinates: 44°41′N 01°10′W / 44.683°N 1.167°W / 44.683; -1.167