Hōsenji Onsen is a hot spring town in Shizuoka, Japan. The water leaves the ground at 17.4°C, cool enough that the baths here are usually reheated. It measures pH 9.5 — strongly alkaline. Officially the water is classed as sulfur spring.
The classic rotten-egg smell and milky white water. It can tarnish silver jewellery, so take it off.
Figures from the town's Japanese Wikipedia article.
34.8272, 138.0144 — open the map
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.