The Hongdo Natural Reserve is located 19 km to the west and made up of about 20 islands, such as Hongdo Island, Tapseom Island, Goyerido Island, Ttiseom Island, and Nopeunseom Island. Hongdo is a small island whose costal line is 20.8 km and is mostly composed of sandstone and quartzite. Because of corrosion by constant waves, broken rock pieces are unparalleled in their grandeur. Hongdo is said to have 545 plants. A primeval forest is found in some parts of the island, which is home to 231 animals including insects, birds, and reptiles. Among them, Precis orithya is rare and can be found only both here and in Jejudo island. The ocean in Hongdo Island is a wonderful outdoor aquarium with clean water and abundant creatures. There are 24 kinds of seaweeds, 117 kinds of invertebrates, and 233 kinds of fish. Since Hongdo Island represents the islands on the southwest and has many unusual aspects, the entire area of Hongdo Island is designated and protected as Natural Reserve.