満天望温泉 is a hot spring town in Oita, Japan. The water leaves the ground at 54.4°C, hot. Officially the water is classed as bicarbonate spring.
Smooth, slightly slippery water. Often nicknamed bihada-no-yu, the water for beautiful skin.
Figures from the town's Japanese Wikipedia article.
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.