Lake Shikotsu is a caldera lake meaning it was formed by a volcano. The name Shikotsu comes from the Ainu language (meaning big hollow). In Japanese "shikotsu" (different kanji ~ 死骨) means dead bones. The government tried to rename the lake but nobody used the new name so they gave up.
The same deep water that makes Lake Shikotsu warm in winter makes it cold in summer. It's a cold lake for a swim.
The Lake Shikotsu Ice Festival on the shores of the lake is one of Japan's best known winter festivals (late January to mid-February). A variety of illuminated snow and ice sculptures are crafted on the shores of the lake.