Manjanggul is a lava tube running under the eastern fields of Jeju, and nothing inside it is limestone. The ledges, the ropes of stone on the floor and the pillar at the far end are all lava, holding the shape they had while they were still moving. Horizontal shelves run along the walls at shoulder and head height, each one a level the river of lava held before it dropped. Following them with your eye is watching the flow fall stage by stage until it drained away to the sea. Partway along lies a slab that drifted here as hardened crust on top of the moving lava and stranded when the flow went out from under it. Islanders noticed long ago that its outline traces Jeju itself, and the resemblance is close enough that you see it before anybody points it out.
At the end of the walkable kilometre a single pillar joins floor to ceiling, 7.6 metres of it, the tallest lava column ever recorded. It built up where lava spilled out of an upper passage through a hole in the roof and froze where it struck. The lighting is just enough to show the dripped texture of it. How old any of this is depends on who you ask. Luminescence dating of the soils buried under these flows came back at about eight thousand years, while the World Heritage listing still carries two to three hundred thousand, so both numbers meet you on the same trip. Beyond the barrier the cave keeps the largest bat colony confirmed in Korea, long-fingered bats packed into the upper level near the third entrance, which stays shut precisely because the life in there is richest. The cave itself has been closed since 2023. It was measured in 1946 by a village schoolteacher who took about thirty of his primary-school pupils into the dark in straw shoes, carrying pine torches. He split them into squads — torch bearers, surveyors, record keepers, porters — mapped the passage his village had always called the manjeongi hole, gave it the name it carries now, and published the measurements.