Kakuma Onsen (Nagano) is a hot spring town in Nagano, Japan. The water leaves the ground at 18°C, cool enough that the baths here are usually reheated. Officially the water is classed as carbon dioxide spring.
Fine bubbles cling to your skin. Rare in Japan, and usually cooler than other springs.
Figures from the town's Japanese Wikipedia article.
36.4531, 138.3642 — open the map
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.