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Setonaikai NP, Hyogo / Tokushima
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A tide-level difference of up to 1.5 m drives water through Naruto Strait at up to 20 km/h; during strong spring tides, whirlpools can reach about 20 m across.
A book printed locally in 1734 reported mermaids in Naruto Strait. Today they belong to folklore; official interpreters suggest witnesses may have seen dugongs or finless porpoises, but neither explanation is proven.
An 8th-century wooden tag found at Nara’s Heijō Palace records a basket of young wakame from Muya Sea in Itano County—today’s Naruto area. It’s an ancient delivery label for a local favorite.
Naruto’s whirlpools can swap sides: as the current reverses roughly every 6 hours with the changing tide, vortices form alternately on the Harima-nada and Kii Channel sides of the strait.
One documented explanation hears “naru” as the roar of rushing water and “to” as a gate or strait. It’s a fitting name for this channel, where the powerful current was once heard before it was seen.
In the early 19th century, Satoura seller Maekawa Buntarō devised ash-dried wakame: coat leaves in plant ash, dry them for 4–5 days, wash, then shade-dry. Some Naruto makers still use an adapted method.
The whirlpools form above a remarkable underwater pinch: Naruto Strait is only 1.3 km wide, with a V-shaped floor about 100 m deep. Fast central water races past slower flows beside the shoals, creating the spin.
Tidal erosion has gouged two giant bowl-shaped hollows, called kaifu, beside the strait: one is more than 160 m deep and the other exceeds 210 m. The whirlpools’ power reaches far beneath the surface.
The whirlpools act like a sea mixer: they help lift and spread nutrients that might otherwise sink. That supports plankton, which draws small fish—and, in turn, larger fish—to these waters.
The early-8th-century Nihon Shoki says the deity Izanagi found Awa-no-to—today’s Naruto Strait—too swift for purification and chose another place. The passage already describes the channel’s powerful current.
In his 1857 print of Awa-Naruto, Hiroshige likened the whirlpools to flowers; Hokusai also drew them. By the late Edo period, this moving seascape had become a celebrated subject for Japan’s great printmakers.
Naruto’s salt-field workers had an annual reward outing called Hamako Yusan: employers served a feast, and the boat trip included watching the strait’s racing currents. A working landscape became a day out.
The future Emperor Taishō watched Naruto’s currents from Chizanmaru, a swift Tokushima daimyo boat built in 1857. The ornate vessel survives and is designated a national Important Cultural Property.
Naruto’s whirlpools appeared on maps before the famous ukiyo-e prints: a Shikoku map from around 1633 is identified as their earliest known map depiction, while a 1691 Japanese map labels them explicitly.
