Seongsan Ilchulbong Tuff Cone rose 180 meters above sea level due to a magma flow under the sea over 5,000 years ago. It was originally a separate island, but a build-up of sand and soil has connected it to the main island. A large crater, formed by the hot lava mixing with cold ocean water, is located at the top of Seongsan Ilchulbong Tuff Cone. The crater is about 600 meters in diameter and 90 meters deep. It has been featured in films, and was originally used for agriculture, but has now been given over to a field of silver grass. From the summit, visitors can see Udo Island, as well as take in the magnificent sunrise. The tuff cone area was designated as a natural reserve on July 19, 2000.