It underwent repeated expansion in the Song and Yuan dynasties. About 3-8 meters below the surface of the downtown area, the tunnel extents some 10 km, it is a military tunnel with the longest history, most complicated structure, largest scale and complete facilities ever discovered in China. Therefore, it is renowned as the Underground Great Wall.

The whole tunnel is crisscrossed with complicated and subtle arrangements, with multilevel distribution and complex construction. The tunnel takes the shape of a “T” when it turns. The two parallel lanes are spaced 2-3.5 meters apart, and there are square holes for passing on a message on the wall between them.

Along the deep and zigzagging tunnel, some military and supporting facilities were built, including foxholes, obstacle walls, stumbling boards, traps, vent holes, holes for passing on a message and niches for torches. The cultural relics of the Han, Tang and Song dynasties unearthed in the tunnel include pills, soldier's swords, lamps, copper mirrors, pottery, porcelain and ink-stones left behind from several dynasties. These are all of great importance for researching China's ancient military buildings and tactics and the military thought of Cao Cao.

Source: Anhui Tour Guide