About two hours up the mountain from Hokodate Trailhead, around Mt. Chōkai 7th Station Ohama Shrine, Chōkai Lake can be seen to the south. The surface of the lake is about 1,600 m above sea level. It is a crater lake with a slightly oblong oval shape from north to south and a diameter of about 200 m.

The crater was formed from volcanic activity about 160 thousand years ago. Lava from that time almost completely covers the mountainside west of here. It is surrounded by two forests on its northeast and southwest, and opens in a horseshoe shape with the trail from Ohama Shrine along its ridge line.

The lake is made up of melted snow, and with a depth of only up to 4.7 m, it freezes completely in the winter. Not many creatures live here due to its acidity (pH 4.7); one species that does is the Japanese black salamander (Hynobius nigrescens).

A lot of snow lingers in this area, and alpine plants bloom as it melts. Long ago, it was called the “sea of birds.”