Karst cave at Jizhou District of north China's Tianjin
Xinhua
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Tourists watch stalactites in a karst cave at Jizhou District of north China's Tianjin on Dec. 5, 2019. (Xinhua/Ma Ping)

Aerial photo taken on Oct. 31, 2019 shows the entrance of a karst cave at Jizhou District of north China's Tianjin. (Xinhua/Ma Ping)

Photo taken on Dec. 5, 2019 shows stalactites in a karst cave at Jizhou District of north China's Tianjin. (Xinhua/Ma Ping)

Photo taken on Dec. 5, 2019 shows stalactites in a karst cave at Jizhou District of north China's Tianjin. (Xinhua/Ma Ping)

Aerial photo taken on Oct. 31, 2019 shows the entrance of a karst cave at Jizhou District of north China's Tianjin. (Xinhua/Ma Ping)