天坑是指具有庞大容积、陡峭而圈闭岩壁的一种特大型喀斯特负地形。发育在厚度巨大、地下水位极深的可溶性岩层中，从地下通往地面，平均宽度与深度均大于100米，底部与地下河相连接。东兰地区有著名的坡锐天坑和仙乐洞天坑，宏伟壮观，美妙绝伦。 Tiankeng refers to the oversize karst negative landform, with huge volume, steep but trapped palisades. It is developed in a tremendous thick soluble stratum with extremely deep underground water. The average width and depth are more than 100 meters from the underground to the ground, with the bottom connected to the underground river. The famous Tiankeng in Donglan are Porui Tiankeng and Xianledong Tiankeng, which are magnificent and wonderful.