Lake Mashu Observatory No. 3 was added to Your Favorites!
Akan-Mashū NP, Hokkaido
Claim this place to tell your story your way.
From this lookout, Kamuy-shu looks like a little island—but only its top 31 m rises above the water. The volcanic pillar continues roughly 240 m down to the lakebed.
At dawn, Pacific sea fog can ride south winds inland and collect in the calderas. From this lookout, it may turn both Lake Mashu and distant Lake Kussharo into separate cloud seas.
Kamuy-shu means “old woman who became a god” in Ainu. Local legend says a grandmother searching for her lost grandchild rested in the lake and became the island.
Mashu’s clear-water ecosystem wasn’t entirely untouched: rainbow trout and crayfish brought from the United States were stocked there, followed later by kokanee. Researchers link the fish-driven shift in plankton to declining transparency.
Lake Mashu fills its own caldera, formed about 7,000 years ago—but that caldera lies on the eastern wall of the much larger Kussharo caldera. Turn around at No. 3 and you can see the neighboring volcanic world.
In 1931, a white disk remained visible 41.6 m below Mashu’s surface—a world-record lake transparency at the time. Its clarity has since declined, but long-term monitoring still ranks it among Japan’s clearest lakes.
No rivers flow into or out of Mashu, making it useful for detecting pollutants delivered through the atmosphere. It is the only Japanese lake registered as a GEMS/Water baseline monitoring station.
There once was a walking trail linking observatories 1 and 3, with a second lookout along the way. The route is no longer maintained and is now impassable—so the numbering hints at a stop you can’t reach by that old path.
In her travel essay about Mashu, novelist Fumiko Hayashi compared Kamuy-shu to a dark mole on the lake’s mirror-smooth surface, while drifting clouds looked to her like a Russian film.
