This stone arch is a naturally-created karstic limestone landform. What is likely to have happened is that there was originally a limestone cave which later collapsed, leaving a part of its ceiling standing, in the shape of an arch. This beautiful as well as original stone arch magnificently frames the landscape lying beyond, dotted with idyllic farm houses at a bank of the Namhangang River. The largest of landscape resources of the arched stone pillar type in Asia, Seongmun is of great scientific interest as well, as it wonderfully showcases the characteristics of karstic formations. A local legend has it that Mother Mago (Mago halmi), a character from shamanistic faiths, used to live under this arch.