Jeju was built by lava flowing out of the sea bed millions of years ago. As the lava cooled, it created the soil, the cliffs, and the strange lava tubes - long underground tunnels - that you can still walk through today. Some Jeju lava tubes are so impressive that UNESCO has made them a World Heritage Site.
The island has its own dialect (a slightly different version of Korean) and its own traditions. The most famous Jeju tradition is the haenyeo (頃措厐) - women divers who swim down 10 metres or more without any breathing equipment to collect shellfish from the seabed. Some of the divers are over 70 years old and have been doing it their whole lives.
Jeju is much warmer than the rest of South Korea. While the north of the country gets snow in winter, Jeju mostly stays green. Mandarin oranges grow here - a sweet, easy-peel fruit - and the island is famous for them. Many South Korean families visit Jeju on holiday the way British families might visit Cornwall.
All over Jeju you'll see funny stone statues called dol hareubang ('stone grandfathers'). They have round bellies, big noses, and hats. People used to put them at the gates of villages to keep watch. Today they are everywhere - in front of cafes, on postcards, and as giant souvenirs you can buy on the way home.