Explore Jeju Island Jeju City Eastern Jeju Seogwipo Jungmun Western Jeju Hallasan National Park Share A short way east of Jeju City, a group of natural attractions provide an enjoyable day-trip. Foremost among them is Manjanggul (만장굴), a long underground cave formed by pyroclastic flows. Underwater eruptions millions of years ago caused channels of surface lava to crust over or burrow into the soft ground, resulting in subterranean tunnels of flowing lava. Once the flow finally stopped, these so-called “lava tubes” remained. Stretching for at least 9km beneath the fields and forests south of the small port of Gimnyeong, Manjanggul is one of the longest such systems in the world, though only 1km or so is open to the public. This dingy and damp “tube” contains a number of hardened, lava features including balls, bridges and an 8m-high pillar at the end of the course.