Mont Saint-Michel used to be connected to the mainland via a thin natural land bridge, which before modernization was covered at high tide and revealed at low tide. Thus, Mont Saint-Michel has been compromised by several developments. Over the centuries, the coastal flats have been polderised to create pasture. Thus the distance between the shore and the south coast of Mont-Saint-Michel has decreased. The Couesnon River has been canalised, reducing the flow of water and thereby encouraging a silting-up of the bay. In 1879, the land bridge was fortified into a true causeway. This prevented the tide from scouring the silt round the mount.
On 16 June 2006, the French prime minister and regional authorities announced a €164 million project (Projet Mont-Saint-Michel) to build a hydraulic dam that will help remove the accumulated silt and make Mont Saint-Michel an island again. It is expected to be completed by 2012.
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, thus ending the trans-channel culture that had stood since the departure of the Romans in AD 460.
Before the construction of the first monastic establishment in the 8th century, the island was called Mont Tombe. According to legend, the archangel Michael 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 Michael burned a hole in the bishop's skull with his finger.
The mount gained strategic significance in 933 when William "Long Sword", 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 duke William of Normandy 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 of Penzance.
During the Hundred Years' War the English made repeated assaults on the island but were unable to seize it, partly because of 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.
The wealth and influence of the abbey extended to many daughter foundations, including St Michael's Mount in Cornwall. However, 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. The Mont Saint-Michel and its bay were added to the UNESCO list of World Heritage Sites in 1979, as they rank very high on such World Heritage Site criteria as cultural, historical, and architectural significance, as well as human-created and natural beauty.
William de Volpiano, the Italian architect who had built the Abbey of Fécamp in Normandy, was chosen as building contractor by Richard II of Normandy in the 11th century. 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 Gothic-style church.
Robert de Thorigny, a great supporter of Henry II of England (who was also Duke of Normandy), reinforced the structure of the buildings and built the main façade of the church in the 12th century. In 1204 the Breton Guy de Thouars, allied to the King of France, undertook the seat of the Mount. After having set fire to the borough and having massacred the population, he was constrained to beat a retreat under the powerful walls of the Abbey. Unfortunately, the fire which he himself lit extended to the buildings, and the roofs fell prey to the flames. Horrified by the cruelty and the exactions of his Breton ally, Philip Augustus offered Abbot Jourdain a grant for the construction of a new Gothic-style architectural set which included the addition of the refectory and cloister.
Charles VI is credited with adding major fortifications to the abbey-mount, building towers, successive courtyards and strengthening the ramparts.
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. The danger of drowning from coastal tides after getting caught in quicksand continues to claim lives.
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 speciality that may be found on the menus of restaurants that depend on income from the many visitors to the mount.
Mont Saint-Michel has also been the subject of traditional, but nowadays good-humoured, rivalry between Normans and Bretons. Bretons claim that, since the Couesnon River marks the traditional boundary between Normandy and Brittany, it is only because the river has altered its course over the centuries that the mount is on the Norman side of the frontier - as expressed in the rhyme "le Couesnon en sa folie mit le Mont en Normandie" ("The Couesnon in its madness placed [Mont Saint-Michel] in Normandy"). This legend amuses the frontier inhabitants who know that the border is not located on the Couesnon river itself but on the firm ground at 4 km in the west, with the foot of the solid mass of Saint-Brelade.