Photo taken on Dec 5, 2016 shows an underground river inside a giant karst sinkhole in Zhenba county of Hanzhong city, Northwest China's Shaanxi province. A cluster of giant karst sinkholes, also known as tiankengs, has been discovered in Shaanxi. In total, 49 tiankengs and over 50 funnels of between 50 to 100 meter in diameter were found in the over 200-km karst landform belt in Hanzhong city. [Photo/Xinhua]