Jeju Volcanic Island and Lava Tubes

Put your palm flat against the nearest stone. It is dark, almost black, and full of tiny holes, like a sponge that has turned to rock. That is basalt, cooled lava, and on Jeju it is everywhere. It is in the low walls between fields, in the harbour breakwaters, and under the soles of your shoes. Before we talk about any history, notice the feel of it. It is rough, light for its size, and faintly warm when the sun has been on it.

Everything around you came out of the ground. Jeju is a volcanic island, built up over a very long time by eruption after eruption. Lava poured out, cooled, and was covered by more lava. In 2007, UNESCO listed part of this landscape as a World Natural Heritage site under the name Jeju Volcanic Island and Lava Tubes. It was the first site in Korea to receive that natural heritage status. The listing is not one place. It is three separate places, spread across the island, that together tell one geological story.

The first is Hallasan, the mountain at the centre of the island. At about 1,950 metres, it is the highest peak in South Korea. On a clear day you can see its long, gentle slopes from almost anywhere on Jeju. It is a shield volcano, which means its sides rise slowly, like an upturned shield, rather than in a steep cone. At the summit sits a crater lake called Baengnokdam, a name often translated as White Deer Lake. Locals tell legends of immortals who came down to drink there.

The second place is Seongsan Ilchulbong, on the eastern tip of the island, within Seogwipo's wider city area. Its name means Sunrise Peak. It is a tuff cone, a volcano formed when hot magma met seawater and exploded into ash and steam. That ash settled and hardened into the layered rock you can see in its cliffs. The crater on top is wide and shallow, ringed by jagged edges. From above, the whole peak looks like an ancient fortress rising straight out of the sea.

Here is something many visitors miss. Look closely at the cliffs of Sunrise Peak, and you will see thin bands of rock stacked like pages in a book. Each band is a record of one pulse of the eruption. Geologists value this cone because those layers are so clear. They show, step by step, how a volcano behaves when it erupts in shallow water. It is a textbook, made of stone, and anyone can read it from the trail.

The third place is underground — the Geomunoreum Lava Tube System. Geomunoreum is a small volcanic hill in the northeast of the island. Long ago, lava flowed out of it toward the coast. As the outer edges of that river of lava cooled and hardened, the hot liquid inside kept moving. When the flow finally drained away, it left behind long, hollow tunnels. These are lava tubes, and the system includes several famous caves, among them Manjanggul, Gimnyeonggul, and Yongcheondonggul.

Manjanggul is the one most visitors walk through. The tube runs for several kilometres, though only a section of roughly one kilometre is open to the public. Step inside, and the air changes at once. It is cool, damp, and still, and the temperature stays fairly steady through the year. Water drips from the ceiling. Your footsteps echo softly. The lighting is low on purpose, to protect the cave, so give your eyes a minute to adjust.

As you walk, look at the walls. You may see flat ledges running along the sides, like shelves. These are flow lines, marks left by lava at different levels as the river inside dropped. On the floor there are ropy ripples, frozen exactly as the molten rock left them. At the end of the public section stands a lava column, a pillar formed when lava poured down from an opening in the ceiling and hardened. It is often described as one of the largest of its kind in the world.

Now for a second surprise. One of the caves in this system, Yongcheondonggul, was found by accident in 2005, when workers were installing utility poles. Inside, explorers found something unusual for a lava tube — white limestone formations and a lake. Shell sand from nearby beaches had been carried down by rainwater over a very long time. The lime in that sand slowly built stalactites, the kind usually found in limestone caves. Because the cave is fragile, it is generally closed to visitors, but its story shows how much may still lie under these fields.

Back above ground, look at the shape of the land. Jeju is covered with small, rounded hills. Locals call them oreum. Each one is a small volcano that erupted on the flanks of Hallasan, and there are well over three hundred of them on the island. Some are grassy and easy to climb. Some hold horses or old family graves inside low stone walls. When you drive across the island, these soft green bumps follow you on every side.

People have shaped their lives around this volcanic ground. The black stone walls you see around fields were built to break the strong island wind and to clear rocks from the soil. Near Sunrise Peak, you may meet the haenyeo, the women divers of Jeju. They dive without oxygen tanks to gather shellfish and seaweed from the rocky seabed. Their tradition is recognised by UNESCO as intangible cultural heritage, meaning a living practice rather than a building or a place.

You will also see the dol hareubang, which means stone grandfather. These are figures carved from the same porous basalt. They have large eyes, wide noses, and hands resting on their bellies. Traditionally they stood at town gates, and people believed they offered protection. Today they appear at museums, hotels, and roadsides. Touch one if you can. The holes in the stone are bubbles of gas, trapped when the lava cooled.

So pause for a moment where you are standing. Whether it is on a trail, at the mouth of a cave, or by a stone wall, the ground under you was once liquid fire. Listen for the wind, which almost never stops on Jeju. Watch for the dark rock against the green fields and the blue sea. As you walk on, keep an eye on the walls, the hills, and the shoreline. The whole island carries the same quiet signature of the volcano that made it.