Shinsuikan

Shinsuikan (神水館) sits by the Kanagawa River in the small Yashio Onsen (八塩温泉) area of Gunma and has been running since 1931, with an interior that still carries a Taisho-era feel. The water is notably salty and a bit slippery on the skin, leaving you with that lingering soft-skin feeling after you get out. The main indoor bath, Oni-no-yu (鬼の湯), has both a heated tub and a cold tub fed straight from the spring at its natural cool temperature. The highlight outside is the cave rotenburo (岩くつ風呂) right on the Kanagawa riverbank, built with local Sanbaseki (三波石) stone. A private reserved open-air bath is available by prior booking at check-in. The inn is a member of Hitou-wo-mamoru-kai (日本秘湯を守る会), the association for hidden hot springs. Overnight stays only; no day-use bathing.