月山志津温泉 is a hot spring town in Yamagata, Japan. 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.
38.4911, 139.9997 — open the map
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.