Three kilometers in circumference, Lake Yunoko was formed when an eruption of Mt. Mitsudake dammed the Yugawa River. A one-hour walking course around the lake allows visitors to enjoy a primeval forest of broadleaf and coniferous trees, wild birds and seasonal flora. The peak season for colorful autumn foliage is from the end of September to the middle of October. Lake Yunoko is part of Okunikko Shitsugen, a registered Ramsar site.