Mount Hallasan is a massive shield volcano which forms the bulk of Jeju Island and is often taken as representing the island itself. There is a local saying stating that "Jeju Island is Hallasan; and Hallasan is Jeju." The mountain can indeed be seen from all places on the island, but its peak is often covered in clouds. The mountain has been designated Korea's Natural Monument no. 182.
The volcanic island was constructed starting in the Pliocene epoch atop the continental shelf, which is about 100 m (300 ft) below sea level in that area. Eruptions of basalt and trachyte lava built the island above sea level, and it now reaches a height of 1,950 metres (6,398 ft). A large volcanic crater over 400 m (1,300 ft) in diameter tops the volcano. About 360 parasitic cones, ororeum in the Jeju dialect, are found on the volcano's flanks. Most of them are cinder cones and scoria cones, but there are also some lava domes and about 20 tuff rings near the coast and offshore, which were formed by underwater phreatic eruptions. The most recent eruptions occurred on the flanks in 1002 and 1007.
There is a crater lake on Hallasan called Baengnokdam, literally "white deer lake." There is a legend attributing the name of the lake to otherworldly men who descend from heaven to play with white deer. Depending on the season, the circumference of the lake is up to 2 kilometres with a depth up to about 100 meters.
The mountain is home to Gwaneumsa, the oldest Buddhist temple on the island. The temple was originally built during the Goryeo Dynasty. Like many other temples in Korea, Gwaneumsa was destroyed and rebuilt in the 20th century. There is a memorial site outside the temple to commemorate the victims of the Jeju uprising that took place between 1948 and 1950. It is one of the most visited places of the island.
There are five hiking trails on Hallasan. They are:
- Gwaneumsa Trail (관음사 탐방로/觀音寺 探訪路) - 8.7 km
- Eorimok Trail (어리목 탐방로/어리목 探訪路) - 4.7 km
- Seongpanak Trail (성판악 탐방로/城板岳 探訪路) - 9.6 km
- Yeongsil Trail (영실 탐방로/靈室 探訪路) - 3.7 km
- Donnaeko Trail (돈내코 탐방로/돈내코探訪路) - 9.1 km
The Donnaeko trail was reopened to the public on December 4, 2009, after a fifteen year hiatus. Only the Gwaneumsa and Seongpanak trails lead to the summit. The Donnaeko, Eorimok and Yeongsil courses only go as far as Witse Oreum, as the rest of the trail leading to the peak has been closed off since 1994 in order to restore and protect the vegetation.
