The 100 m long crater, shaped like a cup, is found on the eastern part of the island. Seawater comes and goes between the crater and the sea through two caves named Cheonjanggul Cave, forming a lake. Cheonjanggul, located at the center of Dongdo, was known as a crater in the past. However, recent studies have revealed that the cave was formed by weathering and erosion. Ecosystem in the areas around Cheonjanngul is well-developed. Spindle trees (Euonymus japonica), which grow at the top of a very steep area, have been designated as Natural Monument (no. 538) in October, 2012.