Seongsan Illchulbong is a distinctive tuff cone produced by three separate hydrovolcanic eruptions on the shallow seabed. The tuff cone is a well-known research site for its hydrovolcanic landforms after its internal structures along the coast were exposed. Seongsan Illchulbong is an archetypical tuff cone (a volcanic edifice created by hydrovolcanic explosion which is at least 50 m high and has steep slopes that are greater than 25 degrees), 179 m high and 600 m in diameter, dominating the eastern seaboard of Jeju Island like a gigantic ancient castle. The tuff cone was produced by a hydrovolcanic eruption upon a shallow seabed about 5,000 years ago when the sea level was identical to that of the present. Seongsan Illchulbong is surrounded by precipitous cliffs except for the western flank (steep side) because the three sides have been eroded by wave action. The tuff cone, therefore, provides superb geological cross-sections of the volcano from the intra-crater deposits to the marginal strata. Diverse geological structures are observable on the sea cliffs, including fractures that were formed during the eruption of the tuff cone and thinly stratified tuff with diverse internal structures. These structures indicate that abundant water has permeated into the volcanic vent (a circular depression in the ground caused by volcanic activity) of Seongsan Illchulbong during its eruption and that the erupted volcanic materials were very wet and sticky. Designated as a natural monument of Korea in 2000 and as a UNESCO’s World Natural Heritage site in 2008, the tuff cone offers excellent scenery and attracts millions of tourists every year.