A monumental undertaking

What's more, combining the 3D images of the Great Wall and the collapsed sections can help researchers find patterns, Li says.

With the help of artificial intelligence, the team found at least 220 hidden doors, or secret passages, along the Great Wall. Li says some of the secret doors have a specific width and height that only allows one person to pass through. These passages are well hidden and soldiers in the past needed to provide a secret code to enter.

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