Katase Onsen is a hot spring town in Shizuoka, Japan. The water leaves the ground at 80–100°C, hot enough that it has to be cooled before anyone can get in. Officially the water is classed as chloride spring.
Salty water. The salt leaves a thin film on the skin, so you stay warm well after leaving the bath.
Figures from the town's Japanese Wikipedia article.
34.8100, 139.0600 — open the map
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.