"We hope that through preventive protection, we can further curb the weathering of stone carvings in Yungang Grottoes, so as to prolong its life and inherit it for a longer time." Yan Hongbin, director of the stone cultural relics protection center of Yungang Research Institute, said. In the summer of Mencius, the Yungang Grottoes in Datong, Shanxi Province, built near the mountain, are magnificent and stand quietly. The first and second grottoes of Yungang Grottoes, the world cultural heritage site known as "cold spring of stone drum", are receiving "daily consultation" from "cultural protection doctors". Yungang Grottoes, founded in the Heping period of the Northern Wei Dynasty, have been built for more than 1500 years. There are 45 main grottoes, a total of more than 1100 small niches and more than 59000 statues. It is one of the largest Grottoes in China. Yungang statues are mostly carved based on sandstone, which has been eroded by the natural environment for thousands of years. The grottoes are faced with rock mass, flood, weathering and other diseases. In recent years, Yungang Research Institute has often carried out disease investigation. The daily maintenance of Yungang Grottoes has become the norm, and the "visits" of "cultural protection doctors" have become frequent. "The first is its own factors. Wuzhou mountain where Yungang Grottoes are located is yellow sandstone, which belongs to lacustrine sedimentation. Due to the sedimentary environment, its texture is uneven, and bedding and fractures are relatively developed." Yan Hongbin introduced that the second is external factors such as temperature and humidity changes, air pollution and precipitation. "Especially in summer, the rock temperature can reach 50 or 60 degrees Celsius. In case of rainfall, it is greatly affected by the sudden drop of temperature." The first and second Grottoes under maintenance are located at the easternmost end of Yungang Grottoes. Due to their similar shape and content, they are called "double Grottoes". Drums can be heard when knocking on the ground in the first grottoes, and there is a thin spring outside the second grottoes. These two grottoes are also collectively called "shigu cold spring". Since November last year, when the stone cultural relics protection center of Yungang Research Institute investigated the current situation of the above two caves, a total of 1095 small diseases were found. There are more than 50000 statues in Yungang Grottoes. How can we quickly find a disease of one or two centimeters? Sun Bo, a staff member of the center, said that during the relevant investigation, the staff will file each disease independently and mark it with a number. It is understood that the common diseases in these caves are mainly warping. "We use water-soluble lime and repair mortar to seal the fire. If warping is accompanied by strong weathering, we also need new materials for pre reinforcement." Yan Hongbin introduced. "Now, the walls of some caves have changed significantly, and some have disappeared. This is the main problem we face." Yan Hongbin believes that daily maintenance can delay this process to a certain extent. After decades of efforts, the stability of Yungang Grottoes has been basically solved, the water damage has been effectively curbed, and the preservation environment and protection status of the Grottoes have been improved. At present, there are 250 monitoring points in Yungang Grottoes to monitor various environmental data in real time. Yan Hongbin said that the next step is to change from rescue protection to preventive protection and research protection. (outlook new era)