Hallasan is one of Korea's three most sacred mountains. It is on Jeju-do, the southernmost island of Republic of Korea (South 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.
There are seven trails on Hallasan.
Hallasan has subtropical plants, warm-climate plants, and subarctic plants. In spring, there are royal azaleas and yellow rape. There are also maple trees. In winter, the mountain is covered with snow. There are deer, which are the symbol of Hallasan.
The ever-changing colors of the scenery make people admire the beauty of nature, and this is why Hallasan was selected as a national park on March 24 1970. All of Jeju Volcanic Island was declared a UNESCO World Heritage Site in 2007. During the last week in January, there is a snow-flower festival in Hallasan.