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瀬戸瀬温泉 is a hot spring town in Hokkaido, Japan. The water leaves the ground at 42.8°C, hot. It measures pH 9.3 — strongly alkaline. Officially the water is classed as simple spring. Its sources put out around 180 litres a minute. 1 inns take visitors here.
Mild and low in dissolved minerals. Usually clear and gentle on the skin, which is why these baths are often recommended for first-timers.
Figures from the town's Japanese Wikipedia article.
43.9394, 143.4292 — open the map
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.