From Wikipedia:

Lake Toya (洞爺湖 / Toya-ko) is a volcanic caldera lake in Shikotsu-Toya National Park, Abuta District, Hokkaido,Japan.
The stratovolcano of Mount Usu lies on the southern rim of the caldera.
It is a nearly circular lake with 10 kilometers diameter in east-west direction and 9km in north-south direction.
The main town is Toyako Onsen (¬}·Ý´ò·Å¬u), on the western shore.
The town Toyako is located on the other side of the lake.

Lake Tˆtya is said to be the northernmost lake in Japan that never ices, and the second most transparent lake in Japan.
Nakano-shima, an island in the middle of the lake, houses the Toya Lake Forest Museum.