Lake Mashū, Volcanic crater lake in Akan-Mashu National Park, Japan

Lake Mashū is a volcanic crater lake in Akan-Mashu National Park with blue waters reaching a depth of 212 meters, surrounded by steep walls that rise 350 meters above the surface. The caldera shows a nearly circular shape with clear shores, while Kamuishu Island stands as the only landmass emerging from the water.

Volcanic activity around the site began about 30,000 years ago, when a stratovolcano roughly 2,000 metres high stood here. The present caldera formed in a massive eruption about 7,000 years ago, one of Japan's youngest caldera eruptions. Around 4,000 years ago eruptions built Mount Kamuinupuri on the eastern rim and the Kamuishu lava dome in the lake centre; the last eruption, a plinian one, came about 2,000 years ago.

The lake's Ainu name is Kintan-kamuy-to, "lake of the mountain god"; a shortened form, Kamuyto, is also used. The name Mashū is of unclear origin, with theories linking it to mas-un-to, "lake of the gulls", though gulls do not live here. Kamuishu Island takes its name from kamuy (god) and shu (old woman), a name given in Ainu yukar oral poetry.

Three observation platforms provide different perspectives of the water, with the western platform offering the widest view and accessible by car from Mashu Station. Fog occurs frequently and can limit visibility, so an early morning visit often provides clearer conditions.

The water has no visible inlets or outlets and maintains its level through underground seepage and rainwater, which contributes to its exceptional clarity. This closed hydrology makes it one of the purest bodies of water in Japan with a visibility depth that once reached 41 meters (135 feet).