Reisenzan Onsen is a hot spring town in Nagano, Japan. The water leaves the ground at 42°C, hot. Officially the water is classed as sulfate spring. 8 inns take visitors here.
Clear water with a faintly bitter taste. Known for holding heat.
Figures from the town's Japanese Wikipedia article.
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.