Hanashiro Onsen is a hot spring town in Gifu, Japan. The water leaves the ground at 17.1°C, cool enough that the baths here are usually reheated. It measures pH 9.6 — strongly alkaline. Officially the water is classed as radioactive spring. Its sources put out around 3 litres a minute.
Contains trace radon that leaves the water quickly. Also called radium or radon springs.
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.