"A line of sky" can often be spotted at famous Chinese mountain sites. This unique landscape is formed when two steep stones stand vertically to each other; the small crack that appears near the bottom is referred to as a "line of sky."

On the famous Huangshan Mountain , the line of sky lies in the middle of Yuping Peak, a key passage leading to the top of the mountain. Beneath the line of sky, the way to the peak is like an alley, with two surrounding "walls" about 30 meters high. There are more than 80 stairs running through the alley, with the widest point reaching two meters, and narrowest, only half a meter, allowing just one person to pass at a time.

The famous line of sky in Yong'an, Fujian Province, is formed in the middle of a crack in a steep stone. The pass beneath it is 80 meters high and 120 meters long, with 206 stairs. The enormous stones on both sides seem to "close in" on visitors as they walk through the long tunnel. Here, only a part of the sky in a lineal shape can be seen. The famous geologist Xu Xiake of the Ming Dynasty (1368-1644) praised the spectacular natural wonder very highly in his book "The Travel Notes of Xu Xiake'.

The line of sky at Wuyi Mountain is about 100 meters long. Its widest part allows only two men to pass at a time, while the narrowest point is only 30 cm wide. While some smaller tourists can squeeze through this point, it is the end of the road for the larger traveler. Noon is the best time to walk through this natural wonder and, on a fine day, visitors will have a chance to enjoy the best of its rare beauty.

At others sites, such as Lingyan Temple in Shandong Province, the line of sky can only accommodate one person. Its surrounding stones are just five meters high and 10 meters long. The line of sky on top of Jiuhua Mountain is actually a vertical crack in granite. It is less than one meter wide and only several meters long. But the most exquisite line of sky can be found on Feilai Peak in Hangzhou, where only a very small fraction of light can be seen.

The spectacle of the sky in a lineal shape has become a red-hot tourist attraction. Some such sites even boast legends about their formation, adding extra glamour and mystery to their splendor.

From the perspective of geology, most of the phenomena appear in the granite conditions, and the grotesque shape of stones is a result of longtime erosion of water that holds carbon dioxide. Some others are generated as a result of geological fault.