In the Naruto Strait, a channel between Naruto of Tokushima Prefecture and Awaji Island of Hyogo Prefecture, large amounts of water are moved to and from the Pacific Ocean and the Seto Inland Sea, creating a phenomenon known as tidal whirlpools. Because the strait is so narrow, the water is pushed through at around 13-15 km/h (8-9 mph), and the tides can create a difference in water level of up to 1.5 m (4 ft 11 in). During the maximum high tides in spring and autumn, the water rushes through even faster, sometimes reaching speeds of 20 km/h (12mph) and creating whirlpools 20 meters (65 feet) in diameter.
These whirlpools are a rare sight and popular tourist destination for people all over the world. They can be seen from the Onaruto Bridge that spans the strait, or there are tourist boats for those who want to experience them up close. The best time to see the whirlpools is 1.5 hours before and after high and low tide.