Mont Saint Michel was previously connected to the mainland via a thin natural causeway, which before modernization was covered at high tide and revealed at low tide. Over the centuries the coastal flats have been enclosed to create pasture and the distance between the shore and the south coast of Mont Saint Michel has decreased. The flow of the Couesnon River which reaches the sea here has reduced, encouraging a silting-up of the bay. In 1879 the establishment of an artificial causeway prevented the tide from scouring the silt round the mount.
In 2006 the French prime minister and regional authorities announced a 164 million Euro project to build a dam using the waters of the river Couesnon and of tides that will help remove the accumulated silt deposited by the uprising tides, and to make Mont Saint Michel an island again. The construction of the dam is now complete. The full project, which is scheduled for compleetion in 2012, also includes the destruction of the causeway and its replacement by a light bridge, under which the waters will flow more freely.
Mont-Saint-Michel was used in the sixth and seventh centuries as an Armorican stronghold of Romano-Breton culture and power, until it was ransacked by the Franks in AD 460.
Before the construction of the first monastic establishment in the 8th century, the island was called "monte tombe". According to legend, St Michael the Archangel appeared to St Aubert, bishop of Avranches, in 708 and instructed him to build a church on the rocky islet. Aubert repeatedly ignored the angel's instruction, until St Michael burned a hole in the bishop's skull with his finger.
The mount gained strategic significance in 933 when William I, Duke of Normandy, annexed the Cotentin peninsula, definitively placing the mount in Normandy. It is depicted in the Bayeux Tapestry, which commemorates the 1066 Norman conquest of England. Ducal patronage financed the spectacular Norman architecture of the abbey in subsequent centuries.
In 1067, the monastery of Mont Saint Michel gave its support to William the Conqueror in his claim to the throne of England. It was rewarded with properties and grounds on the English side of the Channel, including a small island located at the west of Cornwall, which, modelled after the Mount, became a Norman priory named St Michael's Mount.
During the Hundred Years' War the English made repeated assaults on the island but were unable to seize it due to the abbey's improved fortifications. Les Michelettes, two wrought-iron bombards left by the English in their failed 1423–24 siege of Mont Saint Michel, are still displayed near the outer defense wall.
When Louis XI of France founded the Order of Saint Michael in 1496 he intended that the abbey church of Mont Saint Michel be the chapel for the order, but because of its great distance from Paris his intention could never be realized.
Its popularity and prestige as a centre of pilgrimage waned with the Reformation, and by the time of the French Revolution there were scarcely any monks in residence. The abbey was closed and converted into a prison, initially to hold clerical opponents of the republican régime. High-profile political prisoners followed, but by 1836 influential figures, including Victor Hugo, had launched a campaign to restore what was seen as a national architectural treasure. The prison was finally closed in 1863, and the mount was declared a historic monument in 1874. Mont Saint Michel and its bay were added to the UNESCO list of World Heritage Sites in 1979. It was listed with criteria such as cultural, historical, and architectural significance, as well as human-created and natural beauty.
The tides in the area change quickly, and have been described by Victor Hugo as "à la vitesse d'un cheval au galop" or "as swiftly as a galloping horse".
The tides can vary greatly, at roughly 14 metres between high and low water marks. Popularly nicknamed "St Michael in peril of the sea" by medieval pilgrims making their way across the flats, the mount can still pose dangers for visitors who avoid the causeway and attempt the hazardous walk across the sands from the neighbouring coast.
Polderisation and occasional flooding created salt marsh meadows that were found to be ideally suited to grazing sheep. The well-flavoured meat that results from the diet of the sheep in the pré salé (salt meadow) makes agneau de pré-salé (salt meadow lamb), a local specialty that may be found on the menus of restaurants that depend on income from the many visitors to the mount.