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Towada-Hachimantai NP, Akita
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At Ōbuki (大噴), one vent gushes about 9,000 liters a minute at 98°C, forming a roughly 3-meter-wide stream. Its pH is around 1.2—among Japan’s most acidic springs.
A mineral sample was collected here in 1898 as “Shibukuro stone.” When a matching mineral was later identified at Taiwan’s Beitou hot spring, it took Beitou’s name: Hokutolite.
Local lore says hunters found the spring by following a wounded deer, inspiring its old name Shikayu (“Deer Bath”). In 1932, journalist Sugimura Sojinkan is credited with giving it the river-based name Tamagawa.
Akita samurai tried to tame “Tamagawa poison water” in 1841–52 by channeling rain away from the ground. In 1940, acidic spring flow was diverted through Lake Tazawa for power and irrigation; the lake’s native kunimasu disappeared.
At the outdoor rock bath, visitors don’t soak in a pool—they spread mats on geothermally heated ground and wrap up in blankets. Tamagawa is often credited as the birthplace of Japan’s natural ganban’yoku.
The famous pH 1.2 water isn’t the whole story: researchers identified 3 kinds of spring water around Tamagawa, including acidic waters and a neutral spring at pH 6.1. Their chemistry reflects different underground routes.
As spring water cools downstream, minerals settle onto river stones in Hokutolite’s dark-and-white bands, thickening by just 1 millimeter every 10 years. The deposit is protected as a Special Natural Monument.
Here, toji (湯治) means a real stay, not just a soak: the inn describes stays of a week to a month and still has a self-catering wing, communal kitchens, and a dining room for guests.
