The Great South Bay is a lagoon situated between Long Island and Fire Island, in the State of New York. It is about 45 miles (72 km) long and has an average depth of 4 feet 3 inches and is 20 feet at its deepest. It is protected from the Atlantic Ocean by Fire Island, a barrier island, as well as the eastern end of Jones Beach Island and Captree Island. Robert Moses Causeway adjoins the Great South Bay Bridge, which leads to Robert Moses State Park.