Nirasaki Asahi Onsen Nirasaki Asahi No Yu

Nirasaki Asahi no Yu is a small municipal bathhouse with a single indoor bath, no outdoor option. The spring is carbonated and kept flowing straight from the source, unheated and undiluted, so the water in the tub stays naturally lukewarm and tinged faintly green, with fine bubbles that cling to your skin through the whole soak. It's drinkable too, with a mineral, faintly metallic taste, and regulars say a soak here leaves skin noticeably smooth. The building stands alone among rice fields on the edge of Nirasaki, easy to miss if you're not looking for it. There's a tatami rest room and a deck looking out over the mountains, and it's the kind of carbonated spring people hear about from other enthusiasts rather than from a guidebook.