Hallasan, Shield volcano in Jeju Island, South Korea.

Hallasan is a shield volcano that forms most of Jeju Island, with a summit at 1,947 m and a crater lake, Baengnokdam, about 551 m across. The lake sits in a wide green basin ringed by grassy rims; its eastern wall is topped by 2–5 m of basalt lava, while the western wall is trachyte lava dome. About 360 parasitic cones, called oreum, dot the flanks.

Hallasan began forming in the Pliocene epoch on a continental shelf about 100 m below sea level, as basalt and trachyte lava eruptions built the island above the water. Goryeosa records a last eruption in 1007 CE, while geological evidence points to an eruption at least 2,000 years ago. The Hallasan Natural Reserve was designated Natural Monument No. 182 on October 12, 1966, and the national park followed on March 24, 1970.

The name Hallasan comes from a record meaning "to pull down the Milky Way" (雲漢拏引), and it appears in a Goryeo-period record of Choe Yeong and a poem by Yi Saek, so it was in use by the 14th century. Baengnokdam means "white deer lake"; a legend says otherworldly men came down from heaven to play with white deer, and the Sejo Sillok records that Jeju presented a white deer in 1464. A stone altar at Samcheondan, built in 1469 by magistrate Yi Yak-dong, remains from the mountain rites held for the state's peace.

Six trails climb Hallasan. Eorimok (6.8 km) and Yeongsil (5.8 km) stop at Witseoreum, not the summit. Only Gwaneumsa (8.7 km) and Seongpanak (9.5 km) reach Baengnokdam. Donnaeko (7 km) reopened in December 2009 after 15 years but ends at Witseoreum, 1 km below the crater rim via the South Wall junction.

The summit crater, Baengnokdam, is a pit crater rather than a lava-eruption crater: its eastern wall holds a 2–5 m basalt cap over Tamna Formation stream sediments, while the western wall is trachyte lava dome. Its name means "white deer lake", from a legend of otherworldly men playing with white deer; a 1464 Sejo Sillok entry records a white deer presented from Jeju.