Yunoko Lake lies in upper Nikko, formed as a result of an eruption of Mt. Mitsudake. From Yunoko Lake, the Yukawa River flows into Lake Chuzenji. There are also the marshlands of Senjogahara formed when Mt. Nantai erupted and caused the river to dam, and Odashirogahara field, which provides a transition from marshland into grassland. In 2005, the 260.42 hectares of land that include these four locations was registered under the Ramsar Convention on Wetlands as “Oku-Nikko-shitsugen (the marsh in Oku-Nikko).” These marshlands that serve as a natural habitat for living creatures are one of Nikko’s treasures that will be preserved for generations to come.