A sign near the entrance proclaims this seldom-visited attraction a HUMAN FAIRYLAND AND UNDERGROUND PARADISE. Far from it. Aside from odd recent additions, such as a silk factory, these tunnels are dark, damp, and genuinely eerie. A portrait of Mao stands amid murals of ordinary folk "volunteering" to dig tunnels, and fading but catchy slogans (DIG THE TUNNELS DEEP, ACCUMULATE GRAIN, OPPOSE HEGEMONY, and FOR THE PEOPLE: PREPARE FOR WAR, PREPARE FOR FAMINE). Unintentional humor is provided by propaganda posters from the era, which advise citizens to cover their mouths in the event of nuclear, chemical, or biological attack. Built during the 1960s, with border skirmishes with the USSR as the pretext, the tunnels could accommodate all of Beijing's six million inhabitants upon completion -- or so it was boasted. Army engineers were said to have built a secret network of tunnels connecting the residences of Party leaders at Zhong Nan Hai to the Great Hall of the People and the numerous military bases near Ba Da Chu to the west of town. Suspicions were confirmed in 1976 and 1989 when large numbers of troops emerged from the Great Hall of the People to keep the people in check. The construction boom means that this is the only remaining entrance to the nonsecret tunnels; it may disappear soon.