A large crater lake, called Heaven Lake, is in the caldera atop the mountain. The caldera was formed by the VEI 7 Millennium or Tianchi eruption of 946, which erupted about of tephra. This was one of the largest and most violent eruptions in the last 5,000 years (alongside the Minoan eruption, the Hatepe eruption of Lake Taupo in around AD 180, the 1257 eruption of Mount Samalas near Mount Rinjani and the 1815 eruption of Tambora).