《梦溪笔谈·杂志一》
从下面仰望是高岩峭壁，从上面看去则恰与地面相平，以至于诸峰的峰顶也低于山顶之外的地面。
之穴，自下望之，则高岩峭壁；从上观之，适与地平，以至诸峰之顶，亦低于山顶之地面。
If you look up from the bottom, you will see high rocks and cliffs. If you look down from the top, they are just level with the ground. Therefore, the peaks of the mountains are lower than the ground outside the mountain.