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, according to a research team on the Great Wall's defense system.

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.

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 province 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, says Zhang Yukun, a professor from Tianjin University and head of the research team.

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

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.

The most mysterious exits of those secret passages were also found by the team. Such exits were recorded by scholars in the Tang (618-907), Song (960-1279), Ming and Qing (1644-1911) 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," says Li Zhe with the research team.

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