What is "ratonneau"

Ratonneau is one of the islands of the Frioul archipelago in the Mediterranean Sea, off the southern coast of France near Marseille. It is relatively long and thin, approximately 2.5 km long but at most 500 m wide and generally much narrower. It was connected to the nearly island of Pomègues, which runs roughly parallel, by a mole constructed in 1822 to create a port area.