Rising from the sea like something from a fairy tale, Mont-Saint-Michel challenges your sense of reality. An island that’s sometimes not an island. A geological wonder that’s also a geographic oddity. A fortress and an abbey and a UNESCO World Heritage Site. At high tide, the whole place seems to hover over the water, and when it’s out, the seemingly impregnable surroundings become a sandy stretch explored by the curious and the brave. But one thing is certain: When you arrive and see it for the first time—ramparts, stonework, and buildings that seem to grow right out of the granite, all crowned by a soaring steeple capped by an angel—you won’t quite believe your eyes. Situated in the English Channel just off France’s Normandy coast, a visit here will take you back some 1,300 years. It’s currently home to fewer than 50 people, mostly monks and nuns who reside in …