Mont St. Michel, in the north of France. 
You can only reach it by one road which is flooded over during different parts of the day. Even when the tides are low, it is entirely surrounded by quicksand.

Mont St. Michel, in the north of France. 

You can only reach it by one road which is flooded over during different parts of the day. Even when the tides are low, it is entirely surrounded by quicksand.