Mont-Saint-Michel consists of leucogranite which solidified from an underground intrusion of molten magma about 525 million years ago, during the Cambrian period, as one of the younger parts of the Mancellian granitic batholith. These included Lillemer, the Mont Dol, Tombelaine (the island just to the north), and Mont Tombe, later called Mont-Saint-Michel. As sea levels rose, erosion reshaped the coastal landscape, and several outcrops of granite emerged in the bay, having resisted the wear and tear of the ocean better than the surrounding rocks.

Now a rocky tidal island, the Mont occupied dry land in prehistoric times. Over 60 buildings within the commune are protected in France as monuments historiques. It is visited by more than three million people each year. Mont-Saint-Michel and its surrounding bay were inscribed on the UNESCO list of World Heritage Sites in 1979 for its unique aesthetic and importance as a Catholic site. The abbey was used regularly as a prison during the Ancien Régime. Louis XI recognised the benefits of its natural defence and turned it into a prison. A small garrison fended off a full attack by the English in 1433. The island remained unconquered during the Hundred Years' War. The commune's position-on an island just a few hundred metres from land-made it accessible at low tide to the many pilgrims to its abbey, and defensible as the incoming tide stranded, drove off, or drowned would-be assailants. As of 2019, the island had a population of 29. The mainland part of the commune is 393 hectares (971 acres) in area so that the total surface of the commune is 400 hectares (990 acres).

The island lies approximately one kilometre (one-half nautical mile) off France's north-western coast, at the mouth of the Couesnon River near Avranches and is 7 hectares (17 acres) in area. Mont-Saint-Michel ( French pronunciation: Norman: Mont Saint Miché English: Saint Michael's Mount) is a tidal island and mainland commune in Normandy, France. 1 French Land Register data, which excludes lakes, ponds, glaciers > 1 km 2 (0.386 sq mi or 247 acres) and river estuaries.

Rapunzel sun tapestry