As typical lava tubes in Northeastern Jeju-do, Gimnyeonggul and Manjanggul Lava Tubes had originally been connected but were divided into two tubes due to the cave-in. Manjanggul Lava Tube which is one of the largest lava tubes in the world is about 7.4 kilometers long. There are many lava tubes formations such as lava stalactites, lava stalagmites, and lava stone pillars. The tracks of lava flow on the floor serve as valuable data for researching the process of lava tubes formation. On the other hand, Gimnyeonggul Lava Tube is about 700 meters long. The tracks of lava flow remain on its floor, with hardened lava shaped like a waterfall found at the end of the tube. Unlike lime caves, Gimnyeonggul and Manjanggul Lava Tubes in Jeju-do were formed by lava. As these two lave tubes are world-renowned in scale and are valuable data for research, they are designated and protected as a natural monument.