The mountain's caldera was created in 946 by the colossal (VEI 7) Millennium or Tianchi eruption, one of the most violent eruptions in the last 5,000 years, comparable to the 180 AD eruption of Lake Taupo and the 1815 eruption of Mount Tambora. The eruption, whose tephra has been found in the southern part of Hokkaidō, Japan, and as far away as Greenland, destroyed much of the volcano's summit, leaving a caldera that today is filled by Heaven Lake.