The fourth stop in the Geomunoreum system is a quiet one, tucked away in the middle of the island, on the slopes below Hallasan. The volcano's name means roughly the dark hill, and its eruption sent lava flowing down the northern flank to the sea, creating a network of tubes that now form part of the UNESCO site. Not all of them are open to the public, and the quieter ones, in the forest, are of most interest to the hardy, since they preserve the strange, damp, silent world of the island's underground.
The mountain above is the other attraction. Hallasan National Park offers trails through zones of vegetation, from broadleaf forest at the bottom to bamboo grass and dwarf conifers near the top, and a crater lake, Baengnokdam, lies at the summit. The hike to the top is long and strenuous, but on a clear day the view over the island and the sea is excellent. In winter, snow covers the upper slopes and attracts people to the sledding hills, and deer roam in the forest.
For motorcyclists, the road known as the 1100 Road, which crosses the island at a height of about eleven hundred metres, is a classic. It climbs from both sides in a series of gentle curves through woodland, and at its highest point there is a wetland that is protected under the Ramsar convention, with a boardwalk and a rest area. In autumn the trees are red and gold, and in winter ice and snow can close the road, so check conditions. Fog is common and visibility can drop quickly.
The nearest large town is Jeju City, twenty-one kilometres away, with fuel and all services, and there are no stations on the mountain roads, so fill before you climb. Carry a warm layer, since the temperature on the high road can be ten degrees lower than on the coast. Try a bowl of noodles in a roadside restaurant, and watch the clouds roll over the cones.