Kamegai Onsen is a hot spring town in Toyama, Japan. The water leaves the ground at 31.9°C, cool enough that the baths here are usually reheated. It measures pH 9.95 — strongly alkaline. Officially the water is classed as sulfur spring. Its sources put out around 300 litres a minute.
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.
36.5664, 137.3906 — open the map
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.