Geological introduction: Sanmen Sea-Sky Window Cluster integrates mountains, water, caves and sky, which is spectacular, with mysterious underground river resources, wonderful and magnificent karst lakes, karst springs, large karst caves, sinkholes and skylights. All the characteristics of karst landforms such as Tiansheng Bridge and Tiansheng Bridge are concentrated in the Sanmen Sea Scenic Area, forming a veritable Karst World Geopark.