Oyunuma is a gourd-shaped swamp (1 km in circumference) formed after a volcanic reuption created this crater and hot water accumulated at the site.
Such a large scale hot spring swamp is extremely rare in the world and is highly valued academically.
Sulfur spring water as hot as 130 degrees Celsius gushes out from the bottom of the swamp, and clouds of steam hang over the water surface, creating a mysterious scene.
You can peer over the entire swamp from the observatory on the south side.
A popular attraction among visitors is the Oyyama footbath; tourists can bathe their feet in the hot water that flows out of the Oyunuma Swamp. Oyunuma is connected to Jigokudani by a promenade, and takes 30 minutes there and back (a 10 minute drive by car).
