If you pass the north gate of Ganghwa Fortress and follow the path in the forest, you will find O-eup mineral spring. It is a mineral water that was given down from the sky during the construction of the North Gate during the reign of King Gojong of the Goryeo Dynasty as a prayer for rain. When the Mongol army invaded this land, Gojong of Goryeo moved to Ganghwa Island and tried to fight until the end. In preparation for the Mongol invasion, the inner palace, palace, and government office were built over a period of two years in 1232. While the North Gate was being built, it was getting dry and I suffered greatly from thirst. King Gojong built an altar in front of the North Gate and offered a prayer for rain. It is said that the sky began to darken and thunder struck. At that moment, lightning struck a large rock and water came out, which is the mineral water we have today. It is said that everyone who was performing the ritual shed tears at this, so they wrote the character ‘ul-eup’ in ‘5-o’ and called it O-eup (五泣) Yaksu.