Granite islet in the bay of St-Michel, northwest France, situated in the département of Manche near the mouth of the River Couesnon, 24 km/15 mi southeast of Granville. It was converted to a peninsula in 1875 with the construction of a 2 km/1.2 mi long causeway. It has a Benedictine monastery, the abbey being founded in the 10th century in the place of an oratory built in 708.