Photo: User: (WT-shared) Shoestring at wts wikivoyage · CC BY-SA 4.0 · Wikimedia Commons
Ōshima Onsen is a hot spring town in Tokyo, Japan. The water leaves the ground at 49°C, hot. 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.
34.7560, 139.3518 — open the map
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.