Underneath the green hills on the north side of Jeju run some of the longest lava tubes in the world, and Manjanggul is the best known. When the volcano of Geomunoreum erupted, in the distant past, rivers of molten rock flowed north toward the sea, and as the surface cooled and hardened, the liquid inside drained away, leaving long hollow tunnels. The Manjanggul tube runs for more than seven kilometres, and the section open to visitors, about a kilometre, is vast, wide enough for a bus and as tall as a house.
Inside, the air is cool and damp, around eleven degrees even in summer, so bring a jacket. The floor is uneven, rocky and wet, and the walls are streaked with ridges where the lava once flowed, with stalactites of lava hanging from the ceiling. At the end of the route, a lava column nearly eight metres tall stands in the dark, one of the largest in the world. Bats live in the deeper reaches, and the whole place has a hushed, ancient feeling that is hard to forget. Helmets are sometimes provided, and sturdy shoes are essential.
Outside, the land around the caves is the Gotjawal, a tangled forest growing on rough lava terrain, where ferns, vines and evergreen trees thrive in the cracks and hollows. It is a natural reservoir, since rain seeps through the porous rock and feeds the springs that supply the island with water. The ground is too rough to farm, which is why the forest has survived.
The riding here is easy and pleasant. Minor roads run through farmland and woodland, with stone walls and tangerine orchards, and the surface is generally good. There is little traffic outside the peak season, and the views of the coast and the hills are fine. Jeju City, about nineteen kilometres away, has fuel, food and everything else, so fill up there. Check the cave's opening hours and closure days before you go, since they vary by season, and allow about an hour for the visit.