Underneath the black volcanic soil of Jeju's north-east lies a hidden world. The Geomunoreum lava tube system, a group of caves that carry names such as Manjanggul and Gimnyeong, was formed when rivers of molten rock from the Geomun volcano drained away and left hollow tunnels behind them. UNESCO listed the area together with Hallasan and Seongsan Ilchulbong as the Jeju Volcanic Island and Lava Tubes site in 2007, and the tubes are reckoned to be among the finest in the world.
Manjanggul, the best known, runs for several kilometres, though only the first kilometre or so is open to visitors. Inside the air is cool and damp whatever the weather outside, so bring a jacket, and the floor is uneven in places. You walk beneath arched walls streaked with lava ridges, past ropy flow patterns, to a lava column about seven metres tall at the end of the open section. Bats live deeper in, and you will hear water dripping.
The area is wooded in a way the rest of the island is not. These are gotjawal, rough forests grown on jagged lava where nothing could be farmed, a place of ferns, vines and tangled trees that works like a sponge for rainwater and feeds the island's springs. Roads here cross low, quiet country, past stone walls, tangerine orchards and small farms, with Jeju City about twenty kilometres to the west and Seogwipo further south. The wind can be fierce, and fog and rain move in quickly.
The caves have a ticket office and a small museum, with fixed seasonal hours that are worth checking, and some tubes close for conservation. Wear shoes with grip. Fill the tank in Jeju City or the larger villages on the main roads, since the forest lanes have none. Local black pork, grilled over charcoal, and abalone porridge are the island's prized dishes, and small restaurants near the caves serve them. Go in a quiet season if you can, and enjoy the rare feeling of walking inside a volcano's old plumbing.