Near Jigokudani, there is a large pond called Oyunuma. This pond was formed by the eruptions of Mt. Hiyoriyama. The mountain is an active volcano. Smoke is still being released from its slopes and its top.
The water of Oyunuma Pond is 130°C at the bottom. The color is gray-black. It has a sulfurous smell.
There is another pond near Oyunuma. It is much smaller, though. It is called Oku no Yu Pond. The surface temperature of the pond is about 80°C. It is too hot for a bath, but it is relaxing just to watch steam rising from the pond.