Higashi Onsen, Satsuma Iōjima

Iōjima is a small volcanic island in Mishima Village, south of Kagoshima, and Higashi Onsen is about as raw as a bath gets: a few rock pools on the shore directly below a cliff, with waves breaking white around them. The water is a sulfur alum spring and extremely acidic, around pH 1.2 by the village's own figures, among the most acidic baths in Japan, so keep it away from your eyes. The source feeds three bathing spots at different temperatures; the one nearest the spring can be too hot to enter, and the others change as waves wash over them and the tide moves.

It is free, unstaffed and mixed. A stone enclosure above the pools serves as a changing area, though it is sometimes covered by a tarp. Bathers wear whatever suits them, swimsuit, towel or nothing; the village tourism site suggests a swimsuit.

Getting here is the adventure: the Ferry Mishima sails from Kagoshima Port in about four hours on a schedule that changes month to month, at 3,600 yen one way in second class. From the road there is parking for about ten cars and a narrow stepped path down to the water. The downsides are real: no lights, few places to stay, no convenience store, and rough seas can make the pools unsafe.