In Jeju the rivers are dry and that is why the villages are all on the coast
Between 1,502 and 1,990 millimetres of rain a year and almost no watercourse carries water: the basalt swallows it first. It reappears in some 600 springs around the edge, and that is where all the people are.
A map of Jeju shows something odd for a large, mountainous island: the population forms a ring. There is a city on the north coast, a city on the south coast, villages strung all around the edge and, in the middle, practically nothing.
The explanation is neither the scenery nor the gradient, which over much of the island is gentle. It is that in Jeju's interior there is no water on the surface.
01 An island that is a single volcano
Jeju is not an archipelago nor a fragment broken off the continent: it is an entire volcano showing above the sea. At its centre stands Hallasan, 1,947 metres, the highest mountain in South Korea, and from there the ground falls away in every direction as a very shallow shield.
The island measures 1,833 square kilometres, 1.83 per cent of South Korean territory. On that surface there are 368 parasitic cones, oreum in the local speech: the largest number in the world produced by a single volcano.
All that material is basalt, and that is the fact that decides everything else. Jeju's basalt came out in successive flows that cooled full of voids, cracks and galleries. It is an extraordinarily permeable rock.
02 The riverbeds with no water
The immediate consequence surprises anyone arriving with the idea of a rainy subtropical island. Jeju gets plenty of water: 1,502.3 millimetres a year at the Jeju City station and 1,989.6 at Seogwipo, on the south coast. And even so almost no stream carries water.
The watercourses exist, they are mapped and they have bridges over them, but for most of the year they are dry stone gorges. Rain filters away before travelling any appreciable distance on the surface.
That explains something that on other islands would look absurd: here you see a riverbed with no water and, at the same time, dense vegetation and cultivated fields around it. The water is there, but it is not on top.
03 Where the water ends up
Beneath Jeju there is a system of conduits. When a lava flow cools on the outside and keeps moving inside, it leaves an empty tube once it drains. The island is riddled with them, and UNESCO considers the Geomunoreum system the finest lava tube complex in the world.
It is not only a tourist attraction: it is the explanation of the water regime. Between the tubes, the cooling cracks and the porosity of the basalt itself, rainwater goes down fast and accumulates in underground aquifers.
In 2007 Jeju Volcanic Island and Lava Tubes entered the World Heritage list; in 2002 it had been declared a biosphere reserve, and the whole is also a global geopark. The three designations describe the same fact from different angles.
04 And where it comes back out
The groundwater travels until it finds a way out, and it finds it where the aquifer meets the surface: near the sea. Those points are called yongcheonsu in Jeju, spring water, and there are close to six hundred of them on the island.
Many well up literally at the shoreline, some even below the tide line. They were the island's only reliable supply of fresh water for centuries, and they were where they were: down at the perimeter.
The whole human geography follows from this. Villages formed around those springs, and the link was so direct that people who drank from the same yongcheonsu were considered to belong to the same village. It is not a pretty metaphor: it is the practical definition of a community on an island without rivers.
05 The island as a ring
The result is the shape you see on the map. Jeju's 670,000 inhabitants occupy a crown around an empty centre, with Jeju City on the north coast and Seogwipo on the south, separated by the mountain.
That layout also brings a climatic division. The same mountain that empties the centre splits the island into two flanks with different regimes: the south gets almost five hundred millimetres more rain a year than the north, and the north gets the winter snow.
And an agricultural consequence. Without surface water there is no paddy field, the staple crop of the rest of Korea. Jeju grew barley, millet and root crops, and today citrus: an agriculture different from the mainland's for the same reason the villages sit where they sit.
06 How to travel it
The first practical consequence is that the island's natural route is circular. Almost everything worth seeing, and almost everywhere to sleep and eat, is on the coastal ring; the interior is visited by going in and out along spokes.
The second is that it pays to choose a flank. North and south are not interchangeable: rain, temperature and even snow differ. Sleeping always on the same side saves mountain crossings, but leaves out half the island.
And the third changes how you look. If at every empty bridge and dry gorge you ask where the water is, the whole island falls into order: it goes in up top, travels inside and comes out below, and the villages sit exactly at the point where it comes out.
