Oyunuma Ponds - “Great Hot Ponds”
Like nearby Kon-ya Jigoku, the several ponds that comprise Oyunuma developed as the sediment from mud volcanoes and mud pots under the surface of the water was carried up by escaping steam and accumulated to form the banks of each pond.
The eastern side of Oyunuma is no longer active, but the mud volcanoes and mud pots visible beneath the water’s surface are a testament to past activity.
The western side, on the other hand, is still active and exceptionally hot, with a temperature of over 83°C.