A hidden Ming Dynasty (1368-1644) secret door of the Great Wall in North China's Hebei Province Photo: Courtesy of Tianjin University

A new exhibition on the research and digital achievements about the Great Wall kicked off at the Tianjin Museum in North China's Tianjin Municipality on Thursday, revealing more secrets of the great wonder: its hidden doors.

Li Zhe, organizer of the exhibition and one member of the research team, told the Global Times on Thursday that this is the first exhibition showing details about the hidden doors or "secret passages" as they call them.

According to Li, the project was launched by the Tianjin University research team in 2018, and is the world's largest cultural routes digitization project so far, covering a length of 5,500 kilometers of the Great Wall.

The secret door found is a small door opened in a hidden section of the Great Wall according to terrain and military needs, facing outside the pass.

Previously, there were only sporadic researches on secret doors in the academic world. The "secret passages" found on the Great Wall were identified thanks to 130 physical remains of secret doors found through official historical Ming Dynasty (1368-1644) documents.

Zhang Yukun, the leader of the research team and a professor at the School of Architecture of Tianjin University, told the Global Times that the research team has captured more than 2 million images of the Great Wall through ultra-low-altitude drones with centimeter-resolution continuous shooting since 2018, covering 90 percent of the artificial walls along the Great Wall of the Ming Dynasty (1368-1644).

Under the support of ancient maps and advanced technology, the team has confirmed the existence of no less than 220 secret doors.

The Juyongguan Pass of the Great Wall in Beijing, China Photo: Courtesy of Tianjin University

These little-known "secret passages" have played an important role in history.

Li said that some of the secret doors have a specific width and height that only allows one person to pass through. These passages are very well hidden and soldiers needed to provide a secret code to reenter.

"The secret doors hide the secrets of the Great Wall, and also the wisdom of the ancients," Li said, adding that the structure of each secret door is unique, according to the local terrain.

Zhang explained that the Great Wall is not just a "wall." "The Great Wall is not completely closed, but was regularly opened and used for business exchanges. It is more of a channel for communication on both sides." 

According to the official records of the Ming Dynasty, the government allowed nomadic tribes from Northwest China's Qinghai and Hetao, a region with abundant water and grass resources at that time, to graze through secret doors, as evidenced by some of the larger secret doors that could accommodate two horses in both directions. 

This study also discovered the most peculiar secret door in the "secret door family" in North China's Hebei Province. 

According to Li, as early as the Spring and Autumn Period and the Warring States Period more than 2,000 years ago, there were records of facilities similar to the gates, but no corresponding physical evidence has been found. Its side facing the enemy is camouflaged with bricks, and the inner side is actually hollow. The enemy can't tell the position of the entrance from the outside, but when the adjacent main pass is attacked by the enemy, soldiers can break through the entrance from the inside and carry out a surprise attack.

During the research, Li said he sometimes imagined seeing the soldiers of the Ming Dynasty breaking through the hidden doors and rushing out to fight with the enemy at the most urgent time.

"Our knowledge of the Great Wall is far from enough, and the research on secret doors has not yet come to an end." Li said that based on the huge database accumulated so far, the research team will use digital technology to further restore the "true appearance" of the Great Wall, thus presenting a complete Great Wall to the world.