Onneto Yunotaki Falls was added to Your Favorites!
Akan-Mashū NP, Hokkaido
Claim this place to tell your story your way.
Onneto Yunotaki is the largest known place on land where manganese minerals are actively forming. Microbes build more than 1 tonne of black sediment here each year.
Introduced tilapia and guppies ate algae vital to manganese formation; counts topped 15,000. After years of trapping and cooling pools with stream water, their eradication was confirmed in 2019—a Japanese first.
In the 1950s, about 3,500 tonnes of manganese minerals were mined at the falls. Manganese is still important today: it is used in steelmaking and dry-cell batteries.
The manganese-making process here resembles one that began on Earth about 3.5 billion years ago. Scientists value the falls as a natural clue to how ancient oceans and the atmosphere formed.
The hot spring begins as rain on the slopes of Mt. Meakan and Akan-Fuji. It seeps through lava underground for more than 10 years before emerging at a source around 40°C.
Nearby layers dating back 4,000–5,000 years contain manganese minerals. They hint at how today’s soft, microbe-made manganese mud can slowly mature into dense, stable ore.
Visitors once soaked in the pools above the falls. Bathing was prohibited in 1995 to protect the rare manganese-forming environment; the site became a National Natural Monument in 2000.
Microscopic organisms help turn dissolved manganese into the dark coating on the rocks. The spring water carries about 3 parts per million of manganese ions, while microbial mats provide a home for manganese-oxidizing bacteria.
Algae photosynthesize here, but a 2022 study found their oxygen was not driving much manganese formation in the samples studied. Low pH appeared to favor direct oxidation by manganese-oxidizing bacteria.
