幕岩温泉 is a hot spring town in Nagano, Japan. The water leaves the ground at 71.2°C, hot. Officially the water is classed as sulfur spring.
The classic rotten-egg smell and milky white water. It can tarnish silver jewellery, so take it off.
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.