The Bay of Mont-Saint-Michel, its coastal strip, was declared a UNESCO World Heritage Site in 1979 due to its aesthetic value and importance to Christian tradition.

The bay covers an area of about 500 square kilometers, with a second granite islet, Tombelaine, in addition to the island of Mont Saint-Michel.

Due to the significant tidal movements in this region (the largest tidal range in Europe), much of the bay is exposed at low tide. The sea retreats at high speed for more than 10 km, but returns just as quickly.