Namegawa Onsen, Fukushimaya

A proper old-school mountain inn that has kept the same spring for over 200 years across 14 generations. The baths, a mixed open-air rock pool (iwa-buro, 岩風呂), a mixed cypress-wood outdoor bath (hinoki-buro, 檜風呂), and a women-only indoor bath, all flow unheated and undiluted from the source. The water is a milky-white bicarbonate-sulfate spring, and the rock pool with the stream below it is the one to aim for.