Tamagawa Onsen

Tamagawa Onsen sits deep in Towada-Hachimantai National Park, on what's said to be the most strongly acidic hot spring in Japan and the country's largest single-source flow. The water itself looks deceptively mild, clear rather than cloudy and without much smell, but the undiluted source tub stings and tingles on the skin. The indoor bath is built of Aomori hiba wood, chosen for standing up to the acid, and holds eleven separate tubs at different strengths and temperatures: full-strength source water next to diluted and mildly acidic pools, a nuru-yu (ぬる湯) tub for a long soak, a waterfall bath, a jet bath, a box steam bath, and even a tap of diluted source water for drinking. Bathers tend to move between tubs rather than settle into just one. Outside, a paved trail loops past steaming vents and ground heated by the same source, where people lay straw mats directly on the hot rock, the natural ganbanyoku (岩盤浴) the inn is known for; there's also an indoor rock-bath room for overnight guests. Tamagawa has long run as a toji (湯治) inn built for extended, mostly self-catering stays rather than a single night, and it closes each winter, reopening only from late April to late November.