Kusayu Onsen

Kusayu Onsen in Nagano Prefecture flows with mildly acidic thermal waters in a prefectural landscape that is home to Kusatsu Onsen, one of Japan's most celebrated and powerfully acidic hot spring destinations, though the Kusayu name — combining kusa meaning grass or herb with yu meaning hot water — suggests a spring with a distinct identity rooted in the herbal or grassland character of its setting rather than a simple derivative of the Kusatsu name. The acidic mineral character of this spring's water reflects the volcanic geothermal chemistry of Nagano's active mountain systems, where low-pH mineral water has long held a distinctive place in the Japanese tradition of mountain acidic spring bathing. Kusayu Onsen's acidic mineral waters and Nagano setting connect it to the broader Japanese tradition of high-altitude acidic volcanic springs, a distinctive category of thermal water that requires careful approach but rewards bathers with a physically immediate bathing experience of the kind rarely found in the gentler spring types of Japan's lowland thermal landscape.