The Mount was 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 gained a mystical quality, being an island half the time, and being attached to land the other: a tidal island.

However, the insular character of the mount has been compromised by several developments. Over the centuries, the coastal flats have been polderised to create pasture. The coast south of the mount has thus encroached on the distance between the shore and the mount. 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. Now there are plans to remove the causeway and replace it with a bridge and shuttle.

The tides in the area shift 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 tide actually comes in at one meter per second.

The tides can vary greatly, at roughly 14 meters between high and low water marks. Popularly nicknamed "St. Michael in peril of the sea" by mediaeval pilgrims making their way across the tidal 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 dangers from the tides and quick sands continue to claim lives.