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Akayu Onsen is a hot spring town in Yamagata, Japan. It measures pH 7.3–9.5 — close to neutral. Officially the water is classed as chloride spring. Its sources put out around 1 litres a minute.
Salty water. The salt leaves a thin film on the skin, so you stay warm well after leaving the bath.
Figures from the town's Japanese Wikipedia article.
38.0500, 140.1700 — open the map
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.