A central feature of Jeju is Hallasan, the tallest mountain in South Korea and a dormant volcano, which rises 1,950 meters above sea level. The main volcano includes 360 satellite volcanoes. Volcanic activity on Jeju began approximately in the Cretaceous and lasted until the early Tertiary period. The most recent eruptions are estimated to be about 5,000 years ago, which puts the volcano into the active classification, meaning eruptions in the last 10,000 years. The designation as active is not agreed by all, as more monitoring and study are needed to better understand the volcano. The island is covered in volcanic rock and volcanic soil produced by Hallasan (Hangul: 한라산). Baengnokdam (Hangul: 백록담), the crater, and lake in it are located at the peak of Hallasan, which was formed over 25,000 years ago.