The impressive Manjanggul Cave is one of the world's longest lava tubes, a geological remnant left behind when lava deep within the ground rises during an eruption. See remarkable lava stalactites 70 centimeters tall and the famous turtle rock formation that some say is shaped like Jeju Island itself. The tunnel takes roughly 50 minutes to explore.
Formed thousands of years ago by an erupting underwater volcano, Seongsan Ilchulbong (Sunrise Mountain) juts out of the ocean in the shape of a crown. The hike takes 40 minutes (with breaks) and rewards you with a sweeping view of the crater and sparkling east sea.