Beijing: China will soon open to public a 90-km-long "Underground Great Wall" in Hebei Province, which was possibly used as a military defence line.
The "Underground Great Wall", occupying an area of 1,600 square miles is located in Yongqing County. It was built in the Song Dynasty (960-1127) and has been listed as one of China's national protected cultural relics in 2006.
In the Song Dynasty, it was an ancient battlefield between Song and Liao states (916-1125). The ancient underground fortress was found in the 1950s, with sophisticated structures and magnificent scale.
Military establishments like bunkers and living necessities like lamp stands and brick beds which could be heated during winter could be found, China News Service reported.
Experts believe the underground structure used to be a military defence line. Chinese archaeologists said the underground bulwark had similar functions as the Great Wall, so it is also a great discovery in China's archaeological history.