Hallasan (Korean: 한라산) is a shield volcano comprising much of Jeju Island in South Korea.

Hallasan is commonly considered to be one of the three main mountains of South Korea, along with Jirisan and Seoraksan.

In the past, Hallasan has been known by numerous other names in Korean including Buak (부악/釜岳), Wonsan (원산/圓山), Jinsan (진산/鎭山), Seonsan (선산/仙山), Dumuak (두무악/頭無岳), Burasan (부라산/浮羅山), Yeongjusan (영주산/瀛州山), and Hyeolmangbong (혈망봉/穴望峯).

[8] The volcanic island was constructed, starting in the Pliocene epoch, on the continental shelf, which is about 100 m (300 ft) below sea level in the 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).

About 360 parasitic cones, or oreum (오름) in the Jeju language, are found on the volcano's flanks.

[11] 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 (1.2 mi) with a depth up to about 100 metres (330 ft).

[11] In order to understand the tendency of temperature change in Jeju Island, the short-term forecasting model of Halla, the weather data of the Korean peninsula, and the wind vectors from the AWS are analyzed.

[12] The change of the minimum temperature varies depending on the degree of cooling of Hallasan and the intensity of the wind speed.

When the wind blows due to the predominant sea breeze, the air cooled in Hallasan descends to the coastal area and induces a decrease in temperature.

When the synoptic wind blows strongly, the cold air of Hallasan moves to the downwind side and the lowest temperature appears.

[12] When the weak northwestern synoptic winds blow, low temperatures occur in the eastern part of Jeju Island.

The chill of the top of Hallasan descends downhill along the mountain range and is collected in the eastern part of Jeju Island so the morning temperature drops considerably.

50 or more unique species live in Jeju Island, especially in high elevation area in Hallasan.

Ticks live on the mountain and caused much damage to Jeju residents long ago.

According to the world floral region, Hallasan belongs to the temperate subregion with Yangtze River basin and southwest Japan.

Especially, 23 species-specific wild plants designated by the Ministry of Environment (a total of 59 species) are distributed in Jeju Island and 6 of them can be found in the natural reserve area of Hallasan.

Their population has declined due to the decrease in food because of the use of rodenticides and pesticides in recent years and indiscreet fishing.

Roe deer, the largest mammal in Hallasan, was once in danger of extinction, but now its population has increased due to protection policies and much effort by the islanders.

Most amphibians and reptiles on Jeju Island are indigenous animals but there are some invasive species from Japan and China.

Halla is particularly well-suited for amphibians and reptiles as a hideout, with well-developed grasslands and marshlands, and is rich in food.

The Donnaeko, Eorimok and Yeongsil courses only go as far as Witse Oreum (윗세오름), as the rest of the trail leading to the peak has been closed off since 1994 in order to restore and protect the vegetation.

To go to Gwaneumsa Trail by car, you should use road number 1117 which connects Jeju Arra-dong Mountain and Tamra Education Center and Cheon-Wangsa.

To go to Donnaeko Trail by car, you should use road number 1115 which connects Jeju city Araun mountain river, Tamra education center, and Chunwangsa temple.