is a rocky tidal island and a commune in Normandy, located approximately one kilometer off the country's north coast, at the mouth of the Couesnon River near Avranches. The population of the island is 41.

In prehistoric times the bay was land. As sea levels rose, erosion shaped the coastal landscape over millions of years. Several blocks of granite or granulite emerged in the bay, having resisted the wear and tear of the ocean better than the surrounding rocks.