One-line sky introduction: One-line sky is just like its name. The two stones stand side by side, only a few feet apart, with thousands of walls standing on both sides. There is a gap in the stone, which is divided into two from the top to the bottom, just like cutting with a knife and an ax. At the narrowest point, only one person can pass through. Looking up, there is only a line of blue sky in the stone gap, so it is called a line of sky. There are 39 steps in the middle, some are wide and some are narrow, which means there are many hardships and ups and downs.