Photo taken on April 22, 2019 shows a cave of Didonghe Tiankeng, a giant karst sinkhole at Huoshizi Village of Ningqiang County in Hanzhong, northwest China's Shaanxi Province. With a maximum depth of 340 meters, the Didonghe Tiankeng is the largest in Chanjiayan Tiankeng Group in Ningqiang County. (Xinhua/Zhang Bowen)