Mt. Iou  is an active volcano with 512 m in height in Teshikaga town between Lake Mashuand Lake Kussharo . In Akan Mashu National Park, here used to be a sulfur mine, so it has been called Mt. Iou. * The sulfur means “Iou” in Japanese language. But its true name is Mt. Atosanupuri that means “A naked mountain” in Ainu language. A lave dome was born from 1,000 to 1,500 years ago. There are more than 1,500 large and small fumaroles, and those still have been vomiting forth steam. A mysterious view with greenish brown and yellow ground changed colors by sulfur and native light brown ground can be seen, and it smells the sulfur like bad eggs in the air.