A Subterranean Superhighway
The Geomunoreum Lava Tube System is a network of tunnels formed by volcanic activity between 100,000 and 300,000 years ago. An eruption from the Geomunoreum volcano sent basaltic lava flowing 13 kilometers toward the coast. As the surface of the lava cooled and solidified, the molten rock below continued to flow, draining away to leave behind a vast system of empty conduits. This process created more than 20 distinct caves, including Manjanggul Cave, which is one of the largest and most accessible to the public.
The main passage of Manjanggul Cave reaches up to 23 meters high and 18 meters wide. The total length of the cave is 7.4 kilometers, although only a one-kilometer section is open for exploration. Inside, the temperature is a steady 11 to 21 degrees Celsius. The cave system is an important habitat for wildlife, including a colony of at least 30,000 common bent-wing bats, the largest such colony in Korea. It also hosts the Jeju cave-spider. The entire system was designated a UNESCO World Natural Heritage site in 2007.
A Gallery of Molten Rock
The walls and floors of the lava tubes show a variety of geological features, known as speleothems, that document the passage of molten rock. Visitors can observe flowlines on the walls, lava benches, and multi-colored carbonate decorations. The caves contain lava stalactites, stalagmites, and rafts. At the end of the public section of Manjanggul Cave stands a 7.6-meter-high lava column, the tallest known in the world. This feature formed as lava dripped from the ceiling and accumulated on the floor below, eventually joining to create a single pillar.
Other caves in the system possess unique characteristics. Yongcheon Cave, a 2.5-kilometer-long tube, contains both lava formations and secondary carbonate speleothems. Calcium carbonate leaching from the sand dunes above the cave has decorated the interior with calcite soda straws, stalactites, columns, cave corals, and rimstones. This combination of volcanic and limestone cave features is exceptionally rare. Dangcheomuldonggul Cave is also known for its delicate and varied carbonate formations.