Zao Onsen Zenshichi-no-yu

The spring at Zao is among the most acidic in Japan: the water sits around pH 1.3 to 1.6, and it turns a striking milky bluish-white as it hits the air. Zenshichi-no-yu draws directly from this source with 100% kakenagashi (free-flow, no recycling). The main bath is mixed (konyoku) for most of the day, with women-only hours in the afternoon (3-6pm) and swimsuits required outside those hours. There are also private baths for those who want them. The inn sits in Zao Onsen village, one of the major onsen resort areas in the Tohoku region, with a long history going back to a legendary spring discovery around 110 CE.