The magnificent frontier pass, or "the Underground Great Wall", built in the Song Dynasty (960-1127), has been listed as one China's national protected cultural relics in 2006. It will be opened to the public in the near future.

"The Underground Great Wall" is located in Yongqing County, Hebei Province. In the Song Dynasty, it was an ancient battlefield between Song and Liao states (916-1125). The well-known Yang family generals had once fought there. The ancient underground fortress was found there in the 1950s, with sophisticated structures and magnificent scale. Blue bricks for building the pass were also carefully made. Experts believe it must be built under the direction of the Board of Works in the Song Dynasty.

Military establishments like bunkers and living necessities like lamp stands and heatable brick beds can also be found.

According to Yongqing's county annals, part of the county had been occupied by Liao. Experts believe the underground structure used to be a military defense line. The imperial records, folklore and archeological finds all help to judge the underground defense line's current situations. It is about 90 km long, occupying an area of 1,600 sq m.

Chinese archeologists said the underground bulwark had similar functions as the Great Wall, so it is also a great discovery in China's archeological history.