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Akan-Mashū NP, Hokkaido
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Just 1.5 km from Onneto, Yunotaki Falls hosts the world’s largest known on-land manganese-oxide-forming site. Microbes turn manganese in the spring water into black mineral, adding more than 1 tonne each year.
Onneto sometimes “sings” as winter ice forms: eerie pings are generally attributed to thin ice expanding and contracting. In clear freezes, gas bubbles can also be trapped beneath the surface.
Onneto’s water shifted from weakly acidic to pH 8.2 in 2012 without human intervention. Researchers suspect changing stream inputs; a 2023 survey measured it near-neutral, at pH 6.9.
At nearby Yunotaki’s warm pools, released guppies and Nile tilapia grew to more than 15,000. After years of work by Ashoro and park partners, they were eradicated by 2019—the first such wild eradication in Japan.
Onneto formed when lava blocked the Rawan River, backing up water to create the lake. The eruption that built nearby Akan Fuji is thought to have happened roughly 1,000–2,500 years ago.
Onneto’s “Five-Color Pond” nickname has a geological recipe: clear blue water blends visually with yellow lakebed mud, formed partly from iron and aluminum carried in by nearby Nishiki-numa. Depth and sunlight change the hue.
Onneto’s Ainu name is often read as “old/big lake.” Hokkaido’s lake catalog records another interpretation: “parent/larger lake,” paired with nearby tiny Ponto, whose name means “child/small pond.”
At nearby Yunotaki, about 3,500 tonnes of manganese mineral were mined in the 1950s. By around 1964, its roughly 40°C spring pools were popular as a hidden forest bath; bathing ended in 1995.
