Lake Kussharo

Lake Kussharo fills part of the Kussharo caldera, at around 26 by 20 kilometres the largest on land in Japan. The lake itself, about 80 km² in area, is the country's largest caldera lake and the sixth-largest lake overall. Its name derives from the Ainu kutcharo, 'throat', referring to the point where the Kushiro River leaves the lake.

The lake lies in the town of Teshikaga, within Akan-Mashu National Park, a short drive from Lake Mashu. Hot springs rise along its shores – at Sunayu, where warm water seeps through the beach sand, and at the open-air baths of Kotan, Wakoto and Ike-no-yu. In winter, whooper swans gather on the unfrozen water warmed by the springs.

On the lake's south-east shore, the fuming Iōzan (Atosanupuri) and the Kawayu Onsen hot-spring town form part of the same volcanic landscape.

The Kussharo caldera formed through a series of huge eruptions roughly between 400,000 and 30,000 years ago, whose pyroclastic flows spread widely across eastern Hokkaido. Later volcanism produced Nakajima, the large island in the lake, and the Atosanupuri lava domes. Mashu, on the eastern rim, is a younger volcano built on the caldera's edge.

The lake's water was acidified by volcanic activity and an earthquake in 1938, which killed most fish; it has since partly recovered.

Hot springs keep parts of the shoreline free of ice in winter, attracting whooper swans from Siberia. In very cold winters the ice cracks and pushes up into long ridges known as omiwatari. Forests of oak, birch and fir surround the lake.

The lake was part of the Ainu homeland, and the Kotan area on the south shore holds a small Ainu folk museum. Kussharo is also known in Japan for 'Kusshī', a lake monster in the style of Loch Ness that became a popular story in the 1970s.