Tamagawa Onsen Shizen Kenkyuro no Rotenburo

This open-air mixed bath sits right on the nature trail that winds through Tamagawa Onsen's (玉川温泉) volcanic hellscape, fumaroles, boiling mud, and steaming vents on all sides. The water is among the most acidic in Japan at pH 1.2, drawn from the single largest single-source discharge in the country (around 9,000 litres per minute). It's completely exposed along a public walking path, which means hikers pass by, but the setting is unlike anything else. Free to use.