An explosion crater of Mt. Hiyori, Oyunuma is a gourd shaped sulfurous lake approximately one kilometre in circumference. At the bottom of it, hot water bubbles out of a sulfurous hot spring with a temperature of approximately 130°c. Even on the surface, the temperature of the grayish black water is around 40-50°c. In the past, an industry developed around collecting the accumulated sulfur on the bottom of the lake.