The boiling tidal whirlpools of the Naruto Strait in western Japan so celebrated by famed ukiyo-e artist Utagawa Hiroshige (1797-1858) are still a source of wonderment today, partly because this natural phenomenon rarely exists elsewhere.

The water that churns like a washing machine at speeds of up to 20 kph in the 1.3-kilometer-wide strait between Tokushima Prefecture in Shikoku and Awajishima island, Hyogo Prefecture, is a fixture of sightseeing boats that take visitors to the very edge of the vortices.

Hideki Ueshima, a guest professor of ocean environmentology at the Hiroshima Institute of Technology, sits on a committee of academics commissioned in 2014 by authorities in Tokushima and Hyogo prefectures to make an extensive study of the mechanism that produces the phenomenon.

It was part of an effort to verify that the Naruto Strait whirlpools, which can measure up to 20 meters in diameter and more than 1 meter deep, merit UNESCO World Heritage status.

Ueshima said tide levels fluctuate hour by hour due to the gravity of the moon and sun, along with other factors.

He explained that waves spawned once the tidal level in the Kii Channel reaches its zenith move in a northward direction to Osaka Bay due to different sea levels.

This causes waves to rotate counterclockwise around Awajishima island, which is flanked by the Akashi Strait on the main Honshu island side and the Naruto Strait on the other.

It takes five or six hours for the waves generated in the Kii Channel to reach the Harimanada Sea to the north of the island, where they raise the tidal level.

Meanwhile, the tidal level in the Kii Channel drops in that time. The difference at the boundary around the Naruto Strait is up to 1.5 meters.

Given the principle that high-level water always seeks a lower point, a southward current is spawned that rushes through the Naruto Strait from the Harimanada Sea to the Kii Channel.

The water current is so powerful that whirlpools are generated.

After five or six hours, the level of the Harimanada Sea, in turn, becomes lower and that of the Kii Channel rises because the waves around Awajishima island move in a clockwise fashion from the Harimanada Sea to the Kii Channel via Osaka Bay.

Then, in the same way as hours before, but in the opposite direction, the northward current flows through the Naruto Strait, which leads to more swirls.

“The unique geography under the Naruto Strait provides a clue,” said Ueshima.

Both the Harimanada Sea and the Kii Channel have seabed basins that are 140 to 200 meters deep. In addition, the seabed terrain rises up like a wall between the two sea areas.

It is not just the narrowness of the strait, but also the seabed terrain, that raise the difference between the two sea levels.

"A strait with these features is almost unheard of anywhere else in the world,” Ueshima noted.

Numerous other factors are thought to contribute to the phenomenon.

“We have reams of items to research, such as the history of how the Naruto Strait was formed,” said Ueshima.

The council that aims to gain UNESCO World Heritage status for the Naruto Strait whirlpools said aerial footage taken by a helicopter in fiscal 2016 showed that small swirls were spawned one after another, which then merged into larger whirlpools.

Earlier this fiscal year, the council launched drones to take footage of the whirlpools from a lower altitude and released buoys with global positioning system functions to obtain further data, including where swirls occur and the speed they spin.

"We hope the sensation of seeing these beautiful whirlpools will be heightened after, one by one, we unwrap the secrets of the swirls,” Ueshima said.

Uzushio Kagakukan (science museum of whirlpools) located in the Uzunooka Onarutokyo Kinenkan hall, a complex with restaurants and shops in Minami-Awaji, Hyogo Prefecture, is at the forefront of efforts to promote the natural wonder. It reopened in March 2017 after being remodeled.

A new feature at the facility is a large diorama, representing the geography of Awajishima island and its surrounding areas. It is about 5 meters long and 10 meters wide, and filled with water discharged from six pumps installed at the Kii Channel and Harimanada Sea portions of the exhibit.

The hydraulic model supervised by Ueshima is on a scale of 1:2,000 of the geographical area and precisely reproduces the natural tidal phenomenon to help visitors understand how whirlpools emerge in the Naruto Strait.