Kotan-no-yu Open-air Bath

Kotan-no-yu is a free open-air hot spring on the southeastern shore of Lake Kussharo in Teshikaga, eastern Hokkaido, within Akan-Mashu National Park. Its name comes from the Ainu word kotan, meaning village. The stone-lined pool sits almost level with the lake surface, only a few metres from the water, and is cleaned and maintained not by the local government but by resident volunteers. It is open around the clock and free to use.

The water is a sodium bicarbonate spring (Na-HCO3), neutral at pH 7.0, with a total mineral content of 1.056 g/kg. It is pumped from the town-owned Kotan source, drilled to a depth of 100 metres in 1966, at 68.3°C. Nothing is added or removed: no heating, no dilution, no recirculation and no chlorination. The clear water carries a faint oily aroma and a silky feel from its high metasilicic acid content, and the bath itself sits at roughly 41-45°C depending on the season and time of day.

A large rock loosely divides the pool into women's and men's halves, but the two sides remain connected, so bathing is effectively mixed. Unusually for Japan, swimwear, bathing garments and wrapped towels are all permitted, which makes the bath approachable for first-time visitors. Simple log changing huts stand beside the pool, and there is free parking for about five cars along with a public toilet, but no washing area, no showers and no amenities of any kind. Whooper swans gather on the lake directly in front of the bath from late October through April.