Sado-ga-shima (also known as "Sado Island") is a large, S-shaped island off the coast of Niigata Prefecture. After Honshu, Hokkaido, Kyushu, Shikoku, and Okinawa, Sado is Japan's sixth biggest island. The Osado and Kosado mountain ranges, with the Kokunaka Plain wedged between them, make up Sado Island.