Perched atop a rocky island of granite, the castle at Mont Saint-Michel in Normandy, France, towers above the surrounding community. It's located on a tidal island off the country's north coast; at high tide, it's on an island, but at low tide, it's connected to the mainland by a thin strip of land. This siting was advantageous for Mont Saint-Michel when it functioned as a military base and prison in the sixth century. In 1874, it was declared a historic monument and is now owned by the Organization of World Heritage Cities. A project to make Mont Saint-Michel an island again began in 2006 and is expected to be completed in 2015.