Key Largo is an island in the upper Florida Keys archipelago and is the largest section of the Keys, at 33 miles (53 km) long. It is also the northernmost of the Florida Keys in Monroe County, and the northernmost of the Keys connected by U.S. Highway 1 (the Overseas Highway). Its earlier Spanish name was Cayo Largo, meaning Long Small Island.Key Largo is connected to the mainland in Miami-Dade County by two routes. The first route is The Overseas Highway, which is U.S.