Key Largo is a census-designated place in Monroe County, Florida, United States, located on the island of Key Largo in the upper Florida Keys. The name comes from the Spanish Cayo Largo, or "long key". It is the first island of the Florida Keys and the originating point of the Overseas Highway to Key West. It is also the location of the first undersea park in the United States, established in 1963, and called the John Pennekamp Coral Reef State Park (located at Mile Marker 102). The park is primarily offshore and stretches three miles into the Atlantic Ocean and is 25 miles long. Adjacent to this is the Florida Keys National Marine Sanctuary covering 178 square nautical miles. Both areas were designed to protect marine life, including the extensive coral reefs in the area.