It is located in Akan Mashu National Park on the island of Hokkaido, Japan.

Lake Mashū is surrounded by steep crater walls 200 metres (660 ft) high.

[3] The loss in transparency is probably due to the introduction of sockeye salmon and rainbow trout into the lake and landslides.

[6] A local legend says that if a person can see the surface of the lake, they will have bad luck.

Kamuishu Island, a lava dome which rises from the middle of the lake, is one.

The other is Mount Kamui, a stratovolcano with lava dome, which forms the highest point on the eastern shore.

Some rock around the Mashū crater and Mount Nishibetsu is older still, dating from the Middle Pleistocene.

The Mashu-dake Hiking Course is a trail that goes along the crater rim and to the top of Mount Mashū.

[2] The trail leads through forest and grassland for about seven kilometers and takes about 2.5 to 3 hours to hike one way.