A central feature of Jeju is Hallasan, the tallest mountain in South Korea and a dormant volcano, which rises 1,950 meters above sea level. 360 satellite volcanoes are part of the main volcano. Volcanic activity on Jeju began approximately in the Cretaceous and lasted until the early Tertiary period. The last recorded volcanic activity was recorded approximately 800 years ago. The island is covered in volcanic rock and volcanic soil produced by Hallasan. Baengnokdam, the crater and lake in it are located at the peak of Hallasan, was formed over 25,000 years ago while.
Jeju is scientifically valuable for its extensive system of lava tubes (also known as lateral volcanoes or in Korean as Oreum). These natural conduits through which magma once flowed are now empty caves that are some of the largest in the world. The caves provide opportunities for scientific research and are also popular tourist destinations. Off the shores of the city of Seogwipo are a vast belt of pillar-shaped rocks that are examples of the natural beauty of Jeju. Shellfish and animal fossils discovered in this area are also very valuable as scientific resources. Beomseom Island and Moonseom Island, also off the city seacoast, are also well preserved and scenic areas. The variety of animal and plant species on Jeju is also an important reason for its value as a natural reserve. Half of all Korean vascular plants grow naturally on the island while another 200 species of plants indigenous to Korea have been transported here. However, half of these species face extinction. The polar plants which came from the south during a glacial period and inhabit the peak of Jeju is one example. Other plants in the subtropical forest and lower regions of the island are also endangered.
Over 17 mammals, 198 types of birds, 8 types of amphibians, 8 types of reptiles, and 947 insect species have been catalogued in the nature reserve. Endangered species include the Capreolus capreolus pygargus and Felis bengalensis manchuria. Since the island was last connected to the Korean Peninsula 10,000 years ago, animals endemic to the island appeared at that time and this separation from the mainland is also of biological significance.
A famous part of the Mt. Hallasan Nature Reserve is the Pillemot Cave, a site dating to the Paleolithic period. The caves are significant because of the archaeological remains found there. Archaeological evidence from the cave suggests that people have occupied the island since the Paleolithic period.