Lake Mashu

Mashu sits in a caldera with steep walls two to three hundred metres high all round and has no river running in and none running out - it is fed by rain and snow and loses water by seepage and evaporation, which is why almost nothing is carried into it. A transparency measurement of 41.6 metres was recorded in 1931, at the time the highest known in the world; the water is less clear now but still exceptional. Visitors see it from three observation decks on the rim, because descending to the shore is prohibited to protect the water. The other thing to know is the fog: warm summer air over cold water fills the caldera for much of June through August, and a fine clear view is genuinely uncommon in the season most people visit. Local convention says seeing the lake clear brings misfortune, which is a good-natured way of managing expectations.

History

The origin of the name is unclear: the Ainu called the lake Kintan-kamuy-to, "lake of the mountain god", and the researcher Nagata Housei proposed that the Japanese name came from the Ainu Mas-un-to, "lake of the gulls", first rendered as Mashin-ko (Wikipedia, Lake Mashū).

What you'll find

• No inflow, no outflow - a closed basin fed only by rain and snow.
• The clarity - a transparency record set in 1931 that stood as a world figure.
• Three rim decks - the only permitted viewpoints, with the shore closed.
• The fog - filling the caldera through much of the summer.

When to go

June to October; autumn mornings offer better odds of a clear view. Two hours.

What to expect

The lake is often invisible in fog and there is no way to predict it. The shore cannot be reached. There is nothing to do at the decks but look. Winter access depends on road clearing.

Nearby

Lake Kussharo is west; Lake Akan is south-west.

Key facts