Photo taken on June 6, 2018 shows the historical relics site of an underground military tunnel, which was dug along the border between the Northern Song Dynasty (960-1127) and the Liao Dynasty (907-1125) in Xiongxian County, North China's Hebei Province. Its structure and function are so sophisticated that many scholars call it "the underground Great Wall." (Xiongan/Wang Yongkang)