Lake Kuttara is a caldera lake in Shiraoi Town. It is the roundest lake in Japan. It is about 2.5km in diameter and its circumference is about 8km.
Because the lake has no rivers flowing in or out, the water is crystal clear. It has a transparency of 19m.
It is believed that kappa (water imps) live in Lake Kuttara. So there is a Yield sign on the road so that kappa can cross it safely.