Natural patterns on a frozen lake

When Onneto Lake in Hokkaido's Ashoro Town freezes over, some of the ice is filled with bubbles formed by volcanic gas rising from beneath the lake and gas generated from the decomposition of sunken vegetation. This produces beautiful patterns that are a delight for the eyes. With roads closed in winter, the phenomenon on a mostly crystal-clear lake cannot be seen easily and visitors are advised to have an experienced guide and to bring hiking gear.