ZaōOnsen Kawarayu KyōDō Yokujō

One of three public baths at Zao Onsen (蔵王温泉, Zao Onsen), this one has a feature the others don't: the spring water rises directly through a wooden-slat floor beneath you (ashimoto-yushutsu, 足下湧出). The water is a sharp, acidic sulfur spring run as kakenagashi (かけ流し, free-flowing), with well water blended in to bring the temperature down from the hot source. No showers, no soap allowed, no amenities at all. You rinse with a bucket and soak. The building sits over a hot-spring stream, and outside you can see the water flowing underneath. Unmanned facility with a coin box at the entrance.