This is what I found on BBC:
Born from a volcanic eruption around 2,000 years ago, Japan’s mysterious Mashu-ko, or Lake Mashu, is rumoured to be one of the clearest lakes in the world. This mirror-like caldera lake sits within the Akan-Mashu National Park on the island of Hokkaido in northern Japan, and it is considered sacred by Japan’s indigenous Ainu people. Because the lake is surrounded by dense fog and 200m crater walls that drop sharply into the water, visiting Lake Mashu’s shores is strictly prohibited by the Japanese Ministry of Environment for safety reasons. The lake’s inaccessibility not only adds to its ancient intrigue, but also helps preserve its unspoilt waters.
In the Ainu language, Lake Mashu is often referred to as Kamuito, meaning “the lake of the gods”, and locals are known to journey as close to the lake as they can for inspiration during difficult times. The Ainu people believe that a female spirit lives within the lake, and Ainu legend has it that if a visitor sees the surface of its usually mist-covered water, he or she will experience bad luck in one of two ways: men will not be able to get ahead in their careers for a while, and women will not have children until later in life.
The lake is blanketed in fog more than 100 days a year, and at 212m deep, it is one of the deepest lakes in Japan. Because it has no significant inlets or outlets, Lake Mashu has remained pristine for millennia, drawing visitors for hundreds of years. In addition to being known as one of Japan’s most beautiful lakes, Lake Mashu is also among the clearest lakes in the world, with a visibility of 20 to 30m.
See stunning images of the lake below:
Information provided by BBC.
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