Watarase, the nation’s largest reservoir, is nestled within four prefectures: Tochigi, Gunma, Saitama, and Ibaraki. This reservoir was constructed in order to detoxify the mineral poisons, by deposition, flowing out of the old copper mines of Ashio. Encircled by reed grasses, the whole area is a haven for foxes, badgers, wild birds, insects, and other wildlife. Visitors enjoy cycling, fishing, water sports, bird watching, and other activities. The Watarase Reservoir was registered in the Ramsar Convention in 2012.