Tsukahara Onsen Kako-no-Izumi

Tsukahara Onsen Kako-no-Izumi is a strongly acidic hot spring on the mid-slope of Mt. Garan (also called Mt. Iwoyama, 1,045 m) in Tsukahara, Yufuin, Oita Prefecture. Legend holds that the samurai Minamoto no Tametomo discovered it in the late Heian period after watching a wounded deer heal its injuries in the water; the spring has flowed for over 800 years and is counted among Japan's three great medicinal baths.

The water is an acidic, iron-bearing (Fe II/III), aluminium sulphate spring with a pH of about 1.4, making it one of the most acidic hot springs in Japan, second only to Tamagawa Onsen in Akita. Its iron-ion content ranks first nationwide, while its acidity and aluminium-ion content rank second. The source emerges at around 60 degrees Celsius and shifts from clear to a mystical yellow-green as the iron oxidises on contact with air. It is served as free-flowing source water.

The facility offers an indoor bath, an open-air bath, and private family baths, with the open-air bath overlooking the Tsukahara plateau. Nearby is a crater-viewing spot on the mid-slope of Mt. Garan at around 800 m elevation. Admission is 500 yen for adults (indoor bath), 600 yen (open-air), and 2,000 yen per hour for the private bath.