Lake Mashū

Lake Mashu (Mash
6b-ko) is a caldera lake in eastern Hokkaido within Akan-Mashu National Park, Japan, known for very clear water and steep caldera rims. The lake occupies a volcanic crater and is one of Hokkaido’s most visually distinct lakes.

Visitors come mainly for rim viewpoints that look down into the caldera and across the lake rather than for extensive shoreline activities; the lake’s basin sits between steep slopes and offers panoramic observation points. The water has long been noted for its clarity and deep-basin character.

The lake formed in a volcanic caldera; it is managed as part of Akan-Mashu National Park and has been a subject of naturalist interest for its geology and water quality since the modern conservation era.

Geographically the lake lies in eastern Hokkaido; it sits within the protected landscape of Akan-Mashu National Park and is accessed from nearby towns and park roads. Coordinates place it at latitude 43.585, longitude 144.54083.

• Depth and clarity: A caldera lake within Akan-Mashu National Park noted for exceptional water clarity and deep water, with a maximum depth reported around 211 metres.
• Best viewpoints: Popular viewing points are located on the caldera rim, offering panoramic views rather than shoreline access to the water surface.