Tianmen Cliff is a barrier at the northern foot of the main peak in Shennongjia, China. It is called "Central China to Ridge", and there is a magical cave near it at an altitude of about 2200 meters. Because thousands of swallows fly over this cave every day, it is also called swallow cave. The swallow cave is about 20 meters high and its depth cannot be calculated. It is said that so far, no one has reached the end of the cave because it is very wet and cold. Even in the hot summer, the cave is chilly, and it is dark 50 meters into the cave. However, those swallows still fly around. Strangely, swallows never touch people or things when they fly around. The swallows here are different from those in other places. It is short, hemp-colored or dark brown, and its feathers have metallic luster. Why can't it meet people and obstacles in the cave? It turns out that it has a special ability-not to navigate with eyes, but to rely on the "ultrasonic waves" emitted by the mouth.

There are many secrets in Yanzidong, waiting for people to explore and study.