The strait between Naruto and Awaji island has a width of about 1.3 km (0.81 miles).
The strait is one of the connections between the Pacific Ocean and the Inland Sea, a body of water separating Honshū and Shikoku, two of the main islands of Japan.
The tide moves large amounts of water into and out of the Inland Sea twice a day.
During a spring tide, the speed of the water may reach 20 km/h (12 mph), creating vortices up to 20 m (66 ft) in diameter.
The whirlpools can be observed from the shore on Awaji island, from tourist ships, or from the Uzunomichi Walkway of the 1985 Ōnaruto Bridge spanning the strait.