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Shikotsu-Tōya NP., Hokkaido
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Oyunuma’s overflow forms Oyunuma River, which cools as it runs through forest to a natural footbath. It’s a remarkable journey from volcanic hot water to a stream where visitors can soak their feet.
A steam explosion from Hiyoriyama formed Oyunuma as a gourd-shaped crater pond about 1 km around. Its distinctive outline is a trace of explosive volcanic activity.
Sulfur springs at the bottom reach about 130°C, while the surface is about 40–50°C. Hokkaido researchers list a maximum depth of 28 m—a remarkable vertical slice of volcanic heat.
A 1954 Geological Survey report described sulfur mined from Oyunuma as mostly black, sand-sized hollow spheres, with sulfur content above 80%. The pond’s volcanic ore could come ashore as tiny empty beads.
Noboribetsu’s Ainu-rooted name comes from nupur-pet, “dark-colored river,” for hot-spring-clouded water downstream of Oyunuma. In 1790, explorer Mogami Tokunai described the river as swirled white and indigo, too opaque to see its bed.
Miners once floated boats on Oyunuma and bucketed sulfur deposits up from the pond floor. After the area became part of a national park in 1949, tighter restrictions brought sulfur extraction here to an end.
Sulfur once collected from Oyunuma’s floor was used as raw material for yuno-hana bath additive, sold as a Noboribetsu hot-spring souvenir. The pond’s minerals found a second life in local bathtubs.
Oyunuma’s chemistry is as striking as its heat: a 2012 survey measured pH 1.75–2.32 along the western shore. The readings came from field samples of this active volcanic lake.
