The Third Tunnel of Aggression is a tunnel under the border between North Korea (DPRK) and South Korea (ROK), extending south of Panmunjom, Paju City, Gyeonggi Province. It was the third tunnel to be discovered running under the border between the two Koreas.
Only 44 km (27 miles) from Seoul, the tunnel was discovered in October 1978 based on information provided by a defector. It is 1.7 km (1.1 miles) long, 2 m (6.6 ft) high and 2 m (6.6 ft) wide. It runs through bedrock at a depth of about 73 m (240 ft) below ground. It is apparently designed for a surprise attack on Seoul from North Korea, and can easily accommodate 30,000 men per hour along with light weaponry. Upon discovery of the third tunnel, the United Nations Command accused North Korea of threatening the 1953 armistice agreement signed at the end of the Korean War. Its description as a "tunnel of aggression" was given by the South, who considered it an act of aggression on the part of the North.
A total of four tunnels have been discovered so far, but there are believed to be up to twenty more. South Korean and U.S. soldiers regularly drill in the Korean Demilitarized Zone in hopes of finding more.
Initially, North Korea denied building the tunnel. However, observed drill marks for dynamite in the walls point toward South Korea and the tunnel is inclined so that water drains back toward the northern side of the DMZ (and thus out of the way of continued excavation).
North Korea then officially declared it part of a coal mine; black "coal" was painted on the walls by retreating soldiers to help confirm this statement. However, it is claimed that there is no geological likelihood of coal being in the area. The walls of the tunnel where tourists are taken are observably granite, a stone of igneous origin, whereas coal would be found in stone of sedimentary origin.
Photos are forbidden within the tunnel, which is now well guarded, though it is a busy tourist site, where visitors enter by going down a long steep incline that starts in a lobby with a gift shop. The South Koreans have blocked the actual Military Demarcation Line in the tunnel with three concrete barricades. The third is visible by tourists visiting the tunnel and the second is visible through a window in the third.