Lake Kussharo (屈斜路湖, Kussharoko) is the largest caldera lake in the Akan National Park and is the largest lake to completely freeze over in Japan. The name of the lake derives from the Ainu word “kuccharo,” which translates to “the place where a lake becomes a river and the river flows out.” The Kussharo caldera was formed about 30,000 years ago. It used to be twice the size it is today, but the eruptions forming Mount Io and Mount Mashu created the current shape of Lake Kussharo. Lake Kussharo is part of the migrating path of whooper swans. Every winter, an average of 300 whooper swans rest by the sandy beach where geothermal springs prevent it from icing over. Another environmental event occurs in winter called Omiwatari, where the ice forms cracks and is pushed upwards. This is due to the layers of ice that continue to expand and contract during the changes in temperature. Even though this phenomenon happens at other lakes, the largest omiwatari can be seen at Lake Kussharo. There is a legend that says the cracks are a result of male deities walking across the water in search for their bride.