So South Korea has discovered four infiltration tunnels between 1974 and 1990, but they believe there are possibly up to 20 that haven't been found. The 3rd tunnel was discovered in 1978 from a tip from a North Korean defector. 30,000 armed soldiers could have passed through the tunnel in 1 hour and attacked Seoul. North Korea has denied building all the tunnels, saying that they are all old abandoned coal mines. This is preposterous because the tunnels, especially the 3rd one, were made by blasting through straight granite. There is absolutely no coal in this area. From walking in the tunnel I can personally attest that it is straight granite on all sides of you. The walls were painted with coal to make it look like an old coal mine. Also, every few inches were deep holes from the dynamite- they would paint a section yellow, pound a dynamite stick into it, and then blow it up. I saw hundred of these yellow patches with deep holes in them from the dynamite, and they were all angled from the Northern side. The tunnel is about 1700 meters long, and visitors can go through about 360 meters of it. At the end of the 360 meters there is a series of 3 blockades built by South Korea to prevent North Korea from using the tunnel. The tunnel is very deep- 73 meters deep, and to get to it you have to go down this sharply declining tube for like 300 meters. Pictures are forbidden inside the tunnel. It's also really hard to find good pics on the internet- here are some: