On top of the oreum, there is a shallow crater lake which displays a unique example of a wetland.

The geology is mostly composed of water-permeable basalt, so it is unusual to have a lake on top of such volcanic cones.

Suryeong (sea level attitude : 508m) on the Halla mountain in Jeju island.

It is a typical parasitic cone and the mouth of a volcano has a girth of 300m, 40m in depth and 1,000m circumference around the crater.

In the dry season, the crater-lake maintains lower level which makes the area have wetland personalities.

Due to the closed circulation of ecosystem, the wetland is claimed to face easy degradation and extinction.