Shirahone Onsen Yumoto Saito Ryokan

The spring water here turns a cloudy milky white as soon as it meets the air, and Yumoto Saito draws it straight from its own blended source into several baths: a wood-built indoor bath called Ryujin-no-yu (龍神の湯), open-air baths built in the same hinoki and sawara wood, and a couple of rougher, semi-open rotenburo made to recreate the toji (湯治) baths of old. There's also a barrier-free private bath you can book for the family. Reviewers mention a light sulfur smell at the entrance that mostly fades once you're actually in the water, which feels soft and opaque on the skin, and every bath here is kakenagashi, kept flowing fresh from the source. Baths switch between the men's and women's sides at 1am. The ryokan spreads across three wooden wings linked by a covered corridor: Kaizan-so (介山荘), named for novelist Nakazato Kaizan (中里介山), who is said to have worked out the plot of Daibosatsu Toge (大菩薩峠) while staying here; Showa-kan (昭和館), named simply for the era it was built in; and Bokusui-so (牧水荘), named for poet Wakayama Bokusui (若山牧水), who came here for therapeutic bathing and wrote a tanka about the mountains outside. Rooms in Bokusui-so face the river below and pick up the sound of the water. It's an old spring, open some four hundred years, and locals still repeat the old saying that three days in it keeps a cold away for three years.