This file photo taken on Aug. 20, 2018 shows the Melong Tagphug cave site in Ngari Prefecture, southwest China's Tibet Autonomous Region. The Melong Tagphug cave site containing delicate stone tools and pottery shards believed to be at least 4,000 years old was unearthed in Ngari Prefecture of China's Tibet. It is the first prehistoric cave site confirmed on the Qinghai-Tibet Plateau. (Xinhua)