Geomunoreum is a quiet, forested volcanic cone on the north-east side of Jeju, and the real attraction is underground. Thousands of years ago lava poured from this cone down towards the sea, and as the outside of the flow cooled and hardened, the molten rock inside kept draining out, leaving long tunnels. Together they form the Geomunoreum lava tube system, which is part of the UNESCO listing for Jeju's volcanic island and its lava tubes. Manjanggul, the most famous tube, runs for several kilometres, with only a section open to visitors, and it has a lava column rising several metres from the cave floor, a remarkable sight.
Inside, the air is cool all year, so a light jacket feels good in August and the walk is surprisingly easy: wide passage, damp walls, dripping ceilings and the faint blue-black sheen of old lava. The caves are home to bats and, in their deeper parts, small cave-adapted creatures. Above ground the same volcanic soil supports gotjawal, the tangled forest that grows over rough lava ground, where ferns, vines and evergreen trees mix in a way found almost nowhere else.
For riding, this corner of Jeju is calm and green. The roads here run between farms and woods, with occasional views of the sea to the north. Weather can be damp and foggy in the early summer rainy season and windy in the winter. Spring brings yellow fields of canola, autumn gives silver grass on the slopes, and mid-week visits mean far fewer buses. Do not expect twisting mountain roads; this is easy riding with a gentle pulse.
Jeju City is about twenty-five kilometres to the west and is the main place to fill the tank and have a proper meal. Seogwipo on the south coast is farther. Small villages along the way have bakeries and cafes, and the stalls around the lava tube entrance often sell tangerine products and snacks. The ride heads on to Hallasan, whose massive crater lake stands about two thousand metres above sea level, and which is the highest point in all of South Korea, so you will be watching the cone grow larger on the horizon as you go.