Jeju is a volcanic island, dominated by Hallasan: a volcano 1,950 metres (6,400 ft) high and the highest mountain in South Korea. The island measures approximately 73 kilometres (45 mi) across, east to west, and 41 kilometres (25 mi) from north to south.

The island formed by volcanic eruptions approximately two million years ago, during the Pleistocene epoch. The island consists chiefly of basalt and lava.

An area covering about 12% (224 square kilometres or 86 square miles) of Jejudo is known as Gotjawal Forest. This area remained uncultivated until the 21st century, as its base of ‘a’a lava made it difficult to develop for agriculture. Because this forest remained pristine for so long, it has a unique ecology.

The forest is the main source of groundwater and thus the main water source for the half million people of the island, because rainwater penetrates directly into the aquifer through the cracks of the ‘a’a lava under the forest. Gotjawal forest is considered an internationally important wetland under the Ramsar Convention by some researchers because it is the habitat of unique species of plants and is the main source of water for the residents, although to date it has not been declared a Ramsar site.

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