Lake Mashu

One of the world's clearest lakes — a caldera filled to its brim with translucent cobalt water, ringed by sheer 200m cliffs that drop straight into the lake with no shoreline. The Ainu called it "Lake of the Devil" for the impenetrable fog that rolls in from the sea of Okhotsk and swallows it whole. No rivers enter or leave; the water is naturally filtered to near-distilled clarity.

Location

Why Visit

• 1

  Lake Mashu's transparency once measured 41.6 m — among the world's highest ever recorded for a lake

• 2

  The lake sits 200 m below the crater rim; there is no accessible shoreline and swimming is prohibited

• 3

  Fog rolls in from the Okhotsk coast and fills the caldera without warning, erasing the lake from view

• 4

  Local legend says those who see the lake clearly on their first visit will never get married — hence the nickname "mystery lake"

Local Tips

Morning visits before 8:00 give the highest chance of a fog-free view. If the lake is hidden, wait 30–60 min — it often clears suddenly. Combine with Lake Kussharo (15 min by car) where free lakeside hot-spring foot baths are open year-round.