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Akan-Mashū NP, Hokkaido
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In an Ainu legend told about Kamuishu, a grandmother lost her grandson and became the island. Her tears are said to fall as rain or snow when she thinks he has returned.
Kamuishu is not a floating rock: it is the exposed tip of a lava pillar rising about 240 m from the lakebed. Only its top 30 m—and roughly 500 m²—show above the water.
At 210 m below the surface, a 2014 underwater camera found regular marks resembling tire tracks in the sediment. In 2016, researchers retrieved a live crayfish from that same depth.
Mashu originally had no fish. People stocked rainbow trout from 1926, then kokanee salmon in 1968–74; the kokanee ate large water fleas, helping shift the plankton toward smaller species that scatter more light.
In 1931, a 30-cm white disk remained visible 41.6 m below Mashu’s surface—a world-leading lake-clarity reading at the time. Measurements began in 1917, using a method still comparable today.
Three typhoons struck Hokkaido in August 2016, raising Mashu’s water level nearly 1 m. The next month, transparency fell to a record-low 6.3 m; by May 2017, it had rebounded to 26 m.
A few times each summer, dense fog pours over Mashu’s caldera wall like a white waterfall. This dramatic “waterfall fog” is a rush of sea fog carried inland and entering near the rim’s lowest point.
Japan’s only lake in the GEMS/Water baseline network, Mashu is a natural pollution detector: its BHC pesticide levels began falling after China banned the chemical in 1983, despite no nearby major source.
No rivers flow into or out of Mashu, yet a hidden warm spring near its deepest point supplies nearly 90% of the lake’s potassium. Researchers track how seasonal mixing redistributes these minerals.
Though Kamuishu’s visible tip covers only about 500 m², surveys record trees, shrubs and flowers from alpine to coastal plants. Its rocky slopes are difficult to reach, so researchers have not surveyed the whole island.
Lake Mashu sits in a caldera on the eastern edge of the much larger Kussharo caldera, which measures about 26 by 20 km. Mashu’s own basin formed in a major eruption about 7,000 years ago.
Akira Fuse’s hit “Kiri no Mashuko” (“Misty Lake Mashu”) was released on December 1, 1966, and helped make the lake nationally known. Its success even made the signature fog part of the attraction.
