Lake Mashu is located in the eastern part of Akan National Park and is known as a mysterious scenic spot. The "mystery" is attributed to the fact that the lake can be rarely seen in the summer tourist season due to frequent heavy fog. The Environment Agency prohibits entry to the lakeshore area and allows only viewing from observation towers.

It is a beautiful crater lake with its steep encircling wall in complete shape. There is a 300 meter high overhanging cliff in the western part of the crater wall. In the center of the lake there is an oval-shaped small island, Bentenjima (Kamuisshu in Ainu language), 70 m x 50 m in size. The central portion of the island stands 25 m above the water surface. The maximum depth of the lake is 211.5 m and the lake bottom is almost flat and covered by pumice deposits.

There are two inflowing streams, but no observable outlet from the lake exists. It is believed that the lake water seeps out through porous bottom sediments, since the water level remains fairly constant. The lake is oligotrophic and the lake water appears indigo-blue. The transparency of 41.6 m measured in August 31,1931 was said to be the highest in the world, surpassing that of Lake Baikal at that time (40.5 m). The transparency has, however, decreased substantially in recent years (Q).

Lake Mashu is the only lake registered as GEMS/Water Baseline Monitoring Station in Japan. (There are 21 stations registered in Japan including other three types of stations.) The reason for this is, No.1: it is proteced as an national park reserve and hence little anthropogenic disturbance, No.2: the ratio of catchment area to the surface area is relatively small so that the lake could represent wide area airborne pollution, No.3: there’s a whole circulation of the lake water once or twice a year so that the independent analysis of the yearly data could be possible. Currently the Center for Environmental Biology and Ecosystem Studies, the National Institute for Environmental Studies has been the head office for the long-term monitoring project. Please refer to “Links Ein the footer to find Lake Mashu, Long-term Environmental Monitoring site. (2017)