Akaishi Onsen is a hot spring town in Yamanashi, Japan. The water leaves the ground at 12°C, cool enough that the baths here are usually reheated. Officially the water is classed as iron spring and acidic spring. Its sources put out around 5 litres a minute.
Turns reddish brown as the iron meets air. The water is clear at the source.
Strongly acidic, sometimes below pH 2. It stings on cuts and is not for long soaks.
Figures from the town's Japanese Wikipedia article.
35.5664, 138.3928 — open the map
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.