五十沢温泉 is a hot spring town in Niigata, Japan. The water leaves the ground at 50°C, hot. Officially the water is classed as simple spring.
Mild and low in dissolved minerals. Usually clear and gentle on the skin, which is why these baths are often recommended for first-timers.
Figures from the town's Japanese Wikipedia article.
37.0585, 138.9266 — open the map
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.