Ruins of secret passages on Great Wall discovered

This undated photo shows a hidden door of the Jiankou section of the Great Wall in Beijing, capital of China. (Tianjin University/Handout via Xinhua)

TIANJIN, Jan. 4 (Xinhua) -- Ruins of over 130 hidden doors on the Great Wall, one of the world's great wonders, have been recently unveiled through close to centimeter-resolution continuous shooting, said a research team on the Great Wall's defense system on Wednesday.

Based on further picture analysis and field trips to the secret doors, the team found each hidden door designed to be highly compatible with the local topography. In history, those secret passages were for scouts to pass through, while some were built as channels for communication between inside and outside of the Great Wall, or for trade and commerce in ancient times.

This undated photo shows a hidden door of the Huangyaguan section of the Great Wall in north China's Tianjin. (Tianjin University/Handout via Xinhua)

According to some official documents dating back to the Ming Dynasty (1368-1644), nomadic tribes were allowed to use such secret gates for grazing their cattle between northwest China's Qinghai and Hetao, a region with abundant water and grass resources at that time. Such practice is also evidenced by some large secret doors that could allow two horses to pass in both directions, said Zhang Yukun, a professor from Tianjin University, also head of the research team.

"These all help prove that the Great Wall is not completely closed, but 'open' in order," Zhang added.

Previously, China has very few studies on such secret passages. The new discovery can help present a complete and vivid architectural mechanism of the Great Wall.

This drawing shows soldiers in defense breaking the gate from the inside of a hidden door of the Great Wall. (Tianjin University/Handout via Xinhua)

The most mysterious exits of those secret passages were also found by the team. Such exits were recorded by scholars in the Tang, Song, Ming and Qing dynasties. However, never had any physical evidence of such a secret exit been discovered before.

The exit's side facing the enemy is camouflaged with brick, while its side facing the soldiers in defense is built to be hollow. It is almost impossible for the enemy to distinguish the location of the exit from the outside, but when the nearby main pass is attacked, soldiers can break the gate from the inside, like breaking an eggshell, and carry out their surprise attack. "This is a great demonstration of ancient China's military wisdom," said Li Zhe with the research team.

The Great Wall, with a total length of more than 20,000 km, is a UNESCO World Heritage Site and consists of many interconnected walls.