The second-most-visited site in France (after the Eiffel Tower) almost defies description. Mont Saint Michel is a 1,000-year-old engineering marvel, a monastery/abbey that draws tourists today the way it has drawn religious pilgrims for centuries. Now classified as a UNESCO world heritage site, it sits on an island connected to the land by a causeway in a remote part of Northern France surrounded by some of the most extreme tides in the world. To understand its appeal, all you have to do is go -- it's stunning. Just arrive early (or late) in the day and you'll be free to wander around and up and down and appreciate its architectural detail and fascinating history (for a time during the French revolution, it was even used as a prison). If you approach by car, you'll first notice it from about 10 kilometers away, rising surrealistically out of the mist.