The military Demarcation Line (MDL) which winds 241km(155miles) across the Korea Peninsula from East to West is surrounded by the Demilitarized Zone(DMZ). Under the provision of Armistice Agreement(cease-fire) signed on July 1953, both sides pulled back 2km from the last line of military contact(MDL) to insure peace, To watch and guard against, any resumption of hostilities, DMZ is a buffer Zone ceasing all military and hostile actions and well known for it's practical ecosystem.

The 3rd tunnel was discovered on October 17, 1978. It is located 52km from Seoul. It was estimated to take approximately an hour for 10,000 soldiers to arrive Seoul through the tunnel. When this tunnel was first discovered, North Koreans insisted it was made by South Koreans in a plot to invade North Korea. However, this theory proved eventually to be false. Structure of 3rd Tunnel is 1,635 meter-long, 1.95 meters high and 2.1 meters wide, penetrates 435 meters South of Military Demarcation line at a point only 4 kilometers South of the Truce Village of Panmunjom.