Under the northeastern slopes of Jeju there is a hollow world made by fire. Geomunoreum is a small volcanic cone, one of the hundreds that dot the island's flanks, and in the long prehistory of the island it spilled lava that ran downhill in rivers. The surface of each flow cooled and hardened while the molten rock inside kept moving, and when it finally drained away it left long empty tunnels. The Geomunoreum lava tube system, a family of caves including Manjanggul, Gimnyeonggul and others, is among the finest in the world, and it is the core of the Jeju Volcanic Island and Lava Tubes World Heritage site, inscribed in 2007.
Manjanggul is the one you can visit, and the visit is memorable. The tunnel is more than seven kilometres long in total, though only a kilometre or so is open to the public, and walking in feels like entering a giant's drain pipe, thirty metres wide in places and as high as a house. The air is cool all year round, about the temperature of a cellar, so bring a jacket even in August, and wear shoes with grip, as the floor is rough and wet. At the end a tall column of lava, about seven metres high, stands where molten rock once poured down from a hole in the roof. The ripple marks on the walls show the levels of the flow, and tiny bats still use the darker sections.
The roads of north-east Jeju are a pleasure, though they are not mountain roads in the alpine sense. Farm lanes, walled with black basalt, wind through fields of tangerines, canola, and sweet potatoes, and nearby the Bijarim forest holds a grove of ancient nutmeg trees, some well over five hundred years old. Hallasan, the great shield volcano at the island's centre, reaches 1,947 metres, the highest in South Korea. Cross-island roads climb its flanks through deciduous forest and pass through cloud, and these mist-wrapped sections can have a sudden damp chill and low visibility.
Jeju City, about twenty kilometres away, has everything, from fuel and restaurants to bike hire. Wind is a serious factor on the island, especially in spring and autumn, and in summer typhoons can ground ferries and flights. Eat black pork, grilled abalone and local tangerines, and check opening days before you go, because caves can close after heavy rain or for maintenance.