The island is located about one kilometer off the country’s northwestern coast, at the mouth of the Couesnon River near Avranches. The total surface of the commune is 988 acres. As of 2015, the island has a population of 33.
A rocky tidal island, the Mont occupied dry land in prehistoric times. 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.
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.
The Mont has a circumference of about 960 meters and its highest point is 92 meters above sea level. The tides vary greatly, at roughly 14 meters between highest and lowest water marks.
Before the construction of the first monastic establishment in the 8th century, the island was called Mont Tombe. According to a legend, the archangel Michael appeared in 708 to Aubert of Avranches, the bishop of Avranches, and instructed him to build a church on the rocky islet.
In the 11th century, William of Volpiano, the Italian architect who had built Fécamp Abbey in Normandy, was chosen by Richard II, Duke of Normandy, to be the building contractor. He designed the Romanesque church of the abbey, daringly placing the transept crossing at the top of the mount.
Many underground crypts and chapels had to be built to compensate for this weight; these formed the basis for the supportive upward structure that can be seen today. Today Mont Saint-Michel is seen as a building of Romanesque architecture.
Robert de Thorigny, a great supporter of Henry II of England, reinforced the structure of the buildings and built the main façade of the church in the 12th century.
Charles VI is credited with adding major fortifications to the abbey-mount, building towers, successive courtyards, and strengthening the ramparts.