Introduction to the First Line Sky: There is a gap in the stone beam, more than 30 meters long, wide in the south and narrow in the north. It is about 0.5-1 meter high and less than 1 meter deep. The narrow part is only a few inches wide, and only one person can crawl in the middle. The wide part can be passed by one person crawling on the ground. Looking up from the bottom of the cave, you can see that the blue sky outside the mountain is like a line, so it is called "a line of sky".