Hallasan is one of Korea's three most sacred mountains. It is on Jeju-do, the southernmost island of Republic of Korea. It is 1,950m tall. On the top of Hallasan, there is a crater called Baek Rok Dam. It was an active volcano until about 25,000 years ago. It is now dormant. Hallasan is made of basalt, a type of black rock that is produced by volcanoes. It extends east to west. The south side of Hallasan is very steep. The slope of the north side is gentle. The east and south are relatively high but smooth.