Set atop a solid rock in the middle of a bay, the abbey and surrounding town was originally only accessible as the tides allowed. Although subsequent development has allowed 24 hours access to the island, an in-progress dam will restore the rock to its previous state as entirely surrounded by water during high tide.
Regardless of whether there exists a 24 hour bridge to the rock or not, Mont Saint-Michel remains a singularly beautiful structure atop a amazing geological formation. During the day take a tour of the abbey and the (tiny) surrounding town. Even better, stay overnight in one of the hotels and you'll have the run of the grounds as the island empties to only a handful of residents after dark.
The abbey was originally built to the schematics of the Holy Sepulchre in Jerusalem, after both an abbot and a count had had visions of St. Michael. Historians suggest, though, that this was merely a scheme to increase tourism into the area.
This place also is of interest for those of us, who are into the Arthurian mythology, as Thomas Malory claims that this is the place where King Arthur himself battled a virgin devouring giant, before going off on his campaign against the emperor of Rome.