Tourists watch stalactites in a karst cave at Jizhou district of north China's Tianjin on Dec. 5, 2019. [Photo/Xinhua]

Aerial photo taken on Oct. 31, 2019 shows the entrance of a karst cave at Jizhou district of north China's Tianjin. [Photo/Xinhua]

Photo taken on Dec. 5, 2019 shows stalactites in a karst cave at Jizhou district of north China's Tianjin. [Photo/Xinhua]

Photo taken on Dec. 5, 2019 shows stalactites in a karst cave at Jizhou district of north China's Tianjin. [Photo/Xinhua]