Tombelaine, Tidal island in Mont Saint-Michel Bay, France

Tombelaine is a rocky island in Mont Saint-Michel Bay that rises about 45 meters above sea level. It stretches roughly 250 meters in length and displays granite outcrops with ancient geological formations.

English forces established a military outpost here starting in 1423 during the Hundred Years War as part of their operations in the bay. The island served as a strategic point in their broader campaign along the Norman coast.

The name may derive from "Tombe Hélène", after a daughter of King Hoël said to be buried on the rock, or from tumulus Belenis, the tumulus of the Celtic god Belenos. A later local tale tells of a girl named Hélène who died there after losing sight of the ship carrying her lover Montgomery. The priory founded in 1137 was dedicated to Notre-Dame de la Gisante.

Access to the island is banned all year round since 2024, under municipal decree no. 2024/31 of 26 August 2024, because the ruins and the cliff are unstable; the ban lasts until safety work is completed. The island can therefore no longer be reached on foot at low tide, as the bay's more than 10-metre tidal range once allowed. The birds nesting on the island, including brown, great black-backed and herring gulls, can be watched through binoculars from the shore.

The French government purchased the island in 1933 and officially protected it as a historic monument four years later. What many visitors do not realize is that it later became a bird sanctuary, making it an important refuge for seabirds and shorebirds in the region.