: vimarsana.com

This undated photo shows a hidden door of the Jiankou section of the Great Wall in Beijing, capital of China.■　　This undated photo shows a hidden door of the Huangyaguan section of the Great Wall in north China's Tianjin.

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Tianjin , China , Huangyaguan , Qinghai , Guizhou , Great Wall , China General , Beijing , Hetao , Henan , Jiankou , Fujian , Zhang Yukun , Tianjin University Handout , Tianjin University , Xinhua , Ming Dynasty , Heritage Site , Secret Ruins Passages Discovered Great ,