The island was not only dedicated to St Michel, the Celts used it to worship the god Belenus and the Romans built a shrine to Jove. After Julius Caesar's conquest of Gaul, the area became known to the Romans as Armorica, from the Celtic term for “coastal area” including the Brittany peninsula and the territory between the Seine and Loire rivers. From around 350 the Mount was a key port for the trading of tin to the rest of Europe. In 495 a vision of the Archangel of St Michael appeared on the Mount. Between 450 and 500 AD, when the Roman power and population were dwindling, many ships brought fugitives from Britain to Armorica. Around the time of the Roman departure, the Germanic-speaking Anglo-Saxons began a migration to the Eastern coast of Britain, where they established their own kingdoms. The post-Roman Celtic-speakers of Armorica were colonists from Britain, resulting in the Breton language, a language related to Welsh and identical to Cornish in the early period and still used today. They also retained control of Cornwall and Northwest England, where Kingdoms such as Dumnonia and Rheged survived.
The island's cathedral story starts in 709 AD with Bishop Aubert of Avranches. The legend goes that the archangel Michael kept appearing in the bishop's dreams. Michael begged him to build a church on the barren rock known then as Mont Tombe. While the bishop initially had his doubts, he was eventually persuaded to build an oratory on the top of Mont Saint Michel. The mount gained strategic significance in 933 when William “Long Sword”, 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.
It is located approximately one kilometre (just over half a mile) off the north-western coast of Brittany, at the mouth of the Couesnon River near Avranches. The population of the island is 44, as of 2009. The island has held strategic fortifications since ancient times, and since the 8th century AD has been the seat of the monastery from which it draws its name. Over time the bay in which the Mont resides has silted up, hastened by the causeway to the island. On 16 June 2006, the French prime minister and regional authorities announced a €164 million project to build a hydraulic dam using the waters of the river Couesnon and of tides to help remove the accumulated silt deposited by the rising tides, and to make Mont-Saint-Michel an island again. You can see the dam in the above picture. The causeway will be removed to be replaced by a bridge.