Yongchulso Spring, located in Nari, Buk-myun, Ulleung-gun, is a spring that discharges groundwater collected from Nari basin (caldera). The spring discharges 20,000 tons of water a day and maintains an average temperature of 10.2℃. The spring is the main supply of water for the northern areas of Ulleung-do. It is also used as water resource for Chusan hydroelectric power generation plant. The area around Yongchulso Spring is composed of pumiceous and trachytic tuffs. When the caldera was created at Nari basin after the collapse of a volcano, highly permeable (water permeability) pumice deposits formed at the base of the lake. Later, the pumice deposits served as the underground reservoir. Ground-water flows through the highly permeable pumice deposits until the water comes into contact with impermeable trachyte rocks on the outskirts of Nari basin. Then, the water overflows onto the land surface, forming Yongchulso Spring.