Okama Crater

The caldera lake held in the three ridges of Mt. Katta, Mt. Kumano, and Mt. Goshiki in the Zao mountain range got its name because of its shape like a cauldron (kama, in Japanese).

The circumference of the lake is about 1,080 meters with a diameter of about 325 meters. The emerald green surface of the lake is also called a five-color marsh (goshiki-numa, in Japanese) because it changes color depending on the weather and how the sunlight hits the water.
The Zao volcano has erupted 26 times in the past, with the last eruption occurring in 1895. The lake water is highly acidic, preventing living organisms from inhabiting in it.
It is one of the few lakes in the world with thermal stratification, where the water temperature drops down to 2 degrees Celsius a dozen or so meters below the surface but maintains a higher temperature at greater depths.