Jeju, the volcanic island off Korea's southern tip

Jeju lies in the sea south of the Korean peninsula, a short flight from Seoul, and it is quite unlike the mainland. The whole island is volcanic. At its centre rises Hallasan, at nearly two thousand metres the highest mountain in South Korea, often with cloud or snow on its summit. Its lower slopes are scattered with hundreds of smaller volcanic cones, and the coast is a mixture of black lava rock, cliffs and pale beaches. The climate is milder than the mainland's, warm enough for tangerines, which grow all over the island and are sold at every roadside. For generations of Koreans, Jeju has been the honeymoon island and the holiday island. The air route between Seoul and Jeju is one of the busiest in the world.

A volcano and its craters

The island's volcanic landscape is listed by UNESCO as a World Natural Heritage Site, under three headings: Hallasan, the lava tubes, and Seongsan Ilchulbong.

Seongsan Ilchulbong, Sunrise Peak, is the island's emblem. It is a tuff cone rising straight out of the sea at the eastern tip, formed by an underwater eruption, with a great bowl-shaped crater at its top, ringed by sharp rocky points. A stepped path leads up its side, and the view from the rim over the crater and the sea is the classic picture of Jeju. As the name suggests, it is famous for its sunrises, and on New Year's Day crowds climb it in the dark.

The lava tubes are long underground tunnels left by flowing lava. The largest that can be visited, Manjanggul, runs for kilometres under the north-east of the island, with a stretch open to visitors, cool and dark even in summer.

On the south-west coast, the cliffs of the Yongmeori coast are layered sandstone and volcanic tuff, sculpted by the sea into ripples and folds. Above them rises the lava dome of Sanbangsan, with a small Buddhist grotto in its side.

The women divers

Jeju's most famous people are the haenyeo, the women who dive without breathing equipment to gather shellfish, sea urchins and seaweed from the seabed. They hold their breath for a minute or two at a time, and many go on diving into their seventies and beyond. It is a tradition that goes back centuries, and in the twentieth century it made Jeju's women, unusually in Korea, the main earners in many families. Their culture is recognised by UNESCO, but their numbers are falling, and most haenyeo are now elderly. Seeing them at work, in their black wetsuits with orange floats, is one of the sights of the coast.

The island also has its own architecture of basalt. Low walls of black stone divide the fields and protect them from the wind, and the dol hareubang, the "stone grandfathers", carved figures with round eyes and folded hands, stand at the entrances to villages. Seongeup Folk Village keeps a group of thatched houses inside such walls.

Jeju is also an island for walking. The Jeju Olle trails, a network of waymarked routes created from 2007 onwards, run right round the island along coastal paths, farm tracks and village lanes, and inspired similar trails across Korea. Hallasan can be climbed in a long day, through forest and scrub to the crater lake of Baengnokdam near the summit. The summit trails have a daily limit on walkers and must be booked in advance, so it is worth checking before you go.

The tours that cross to Jeju

G Adventures' Essential Korea, twelve nights, includes Jeju, along with Seoul, Gyeongju, Jeonju, Busan, the green tea fields of Boseong, the bamboo of Damyang and the port of Yeosu.

G Adventures' Journeys: Soul of South Korea, eight nights, includes Seongsan Sunrise Peak on Jeju, along with Seoul, Gyeongju, Yangdong and Busan. G Adventures' Seoul to Tokyo: Korea and Japan Highlights, fourteen nights, spends time on Jeju at Seongsan, Seongeup Folk Village, the Yongmeori coast, Sanbangsan, Cheonjiyeon waterfall and the clear waters of the Soesokkak estuary, before going on to Japan.

Spring and autumn are the best times to go, with rapeseed and cherry blossom in spring and clear skies in autumn. Summer is hot, humid and busy, and the island is in the path of the occasional typhoon.