Drones uncover the mystery of the Great Wall's 'hidden doors'

For more: https://www.cgtn.com/video Using cutting-edge drone technology, a team of experts from Tianjin University have made a stunning discovery: over 200 doors, gateways and entrances hidden in the Ming Great Wall. What's more, one of these is a "tumen," a special type of hidden door designed by the philosopher Mozi thousands of years ago in the Spring and Autumn Period. Subscribe to us on YouTube: https://goo.gl/lP12gA Download our APP on Apple Store (iOS): https://itunes.apple.com/us/app/cctvnews-app/id922456579?l=zh&ls=1&mt=8 Download our APP on Google Play (Android): https://play.google.com/store/apps/details?id=com.imib.cctv