Manjanggul ends at a lava column 7.6 metres tall, the tallest ever recorded in any lava cave. It grew where molten rock from an upper passage poured through a hole onto the floor below and hardened where it landed, welding ceiling to ground. The tube it stands in runs about 7.4 kilometres under the fields of eastern Jeju, in places 23 metres wide and 30 metres high, and belongs to the Geomunoreum lava tube system that UNESCO inscribed in 2007. Even its age is unsettled: the heritage listing still gives 200,000 to 300,000 years, while recent luminescence dating of Geomunoreum, the cone that fed it, puts the eruption nearer 8,000 years — young enough that people were already living on Jeju when the lava ran.
One kilometre is open, walked out and back the same way. The floor is a level deck now, the lighting deliberately dim so algae cannot take hold on the rock, and the air holds around 13°C whatever August is doing above — the cold hits you within a few steps. Ledges and ropy lines along the walls mark the heights the lava river stood at before it drained away, and halfway in sits Turtle Rock, a stranded raft of crust whose outline islanders read as Jeju itself. Korea's largest confirmed bat colony roosts deep in the unlit reaches, behind the barrier. The cave was closed in December 2023 after rock fell at the entrance, where freezing and thawing keeps working on the roof; it reopened on 30 May 2026 with a rebuilt walkway and steel supports.