South Korea has discovered tunnels that cross the DMZ. To build these tunnels, explosives were used. This left certain traces, called "blasting lines". The orientation of these blasting lines shows that the tunnels were built from north to south, North Korea built them. In total, four tunnels have been discovered, the first on November 15, 1974. North Korea claimed that the tunnels were used for coal mining. No coal has been found in the tunnels. All tunnels were dug in a rock called granite. Some of the tunnel walls have been painted black to give the appearance of anthracite.