Zhangjiajie Tianmen Cave.

Tianmen Cave is located halfway up Tianmen Mountain in Zhangjiajie. It is an underground karst cave on the edge of a platform. Due to the pressure, collapse, and erosion of the syncline structure, the rock layers at the north and south ends collapsed, exposing the cave, which is called "Tianmen Cave." The cave is 131.5 meters high, 57 meters wide, and 60 meters deep. It is located on the north and south sides of the mountain, and the weather changes frequently. It looks like a mirror embedded in the sky, surrounded by clouds and mist all year round. It is a rare high-altitude natural karst cave in the world.