Lake Mashu, Japan: One of the Clearest Lakes in the World

Lake Mashu is an endorheic crater lake located in Akan National Park on the island of Hokkaido in Japan. It is mostly known for its exceptional clarity, and is often referred to as the clearest lake in the World.

Lake Mashū Stats

Lake NameLake Mashū
Surface area18.740
Maximum depth212.0
Average depth117.3
Lake typeCrater lake
Catchment area40.70
Inflowstwo streams
Shore length19.60
Residence time29816.5
FrozenDecember to April
Trophic stateOligotrophic
Average discharge0.853
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Geography and Geology

The lake is located in a volcanic caldera which formed 32 000 years ago and which is still considered potentially active today. It lies in the remains of a stratovolcano, which is basically a parasitic cone of the much larger Lake Kussharo caldera. The eruption that caused the caldera of Mashu occurred 70 000 years ago. The last eruption happened merely 2000 years ago, when pumice was dropped over the region.

The Mashu volcano is rated with a volcanic explosivity index of 6, which is the second highest rating among large volcanoes of this kind. The caldera is home to two volcanoes. Mount Kamui is a stratovolcano with a lava dome, with the highest point of the region, located on the eastern shore of the lake. Kamuishu Island is another lava dome which rises from the center of the lake.

The oldest rocks around Mashu crater date back to the Middle Pleistocene. The majority of the rocks found in the area are andesite and dacite. The rocks here are non-alkali pyroclastic flow on mafic rock, dating from the Late Pleistocene to the Holocene.

There are no settlements along the lake’s coastline. The closest and largest settlement is located 15 kilometers south of the lake. Akan National Park is made up of two other lakes besides Mashu (located to the East), called Akan (to the West) and Kussharo, between the two.


The lake doesn’t have any significant inlets, or outlets. With a maximum depth of nearly 212 meters, it is the deepest lake in the country. Its record transparency was made public after a measuring on the 1st of August, 1931, when professionals proclaimed a transparency of 41.6 meters. Since the ‘50s, its transparency is located somewhere between 20 and 30 meters. This loss is mainly due to the introduction of the sockeye salmon and the rainbow trout.

During the summer months the lake surface is often shadowed by fog, giving the lake a mystical reputation. Local legend has it that if one can see the lake’s surface during a clear day, he will encounter bad luck.

Flora and Fauna

Lake Mashu is home to many phytoplankton and zooplankton. Sockeye salmon and rainbow trout are two introduced fish species. The slopes around and above the lake contain an amalgam of birch forests and evergreen forests.


The lake was originally named the the “Lake of the Devil” by the Ainu people. The Japanese converted its name into “Mashin”, and started referring to the lake as “Lake Mashin”. It got its current name from the surrounding peak of Mount Mashu. The Ainu name, “Kamuito” is also often used today, translated as “mountain of the legends”.

Visiting the Lake

Entrance to the lake shores is strictly prohibited by the Japanese Ministry of Environment. Those who would like to visit the lake and admire its pristine beauty can access two viewing platforms. Platform 1 is larger and busier, located in the South, whilst Platform 3 is lesser-known and more likely to be less crowded, located in the North. These observation decks are suspended on high cliffs above Lake Mashu.

Nearby attractions consist of the Kushiro River, ideal for canoeing, the hot spring resorts of Kawayu and Akan Kohan, and the many excellent hiking trails, including Mount Meakan.

Lake Mashū Map