This natural phenomenon occurs in the Detroit of Naruto, between the islands of Shikoku and Awaji-shima, due to the difference in water level between the Inland Sea and the Pacific Ocean. Whirlpools that can measure up to 20m in diameter in spring and autumn are formed at each tide. They can be seen from the glazed observation platform on Onaruto Bridge or from a boat (20-30min cruises).