Upon their discovery , North Korea claimed that the tunnels were for coal mining ; however , no coal has been found in the tunnels , which are dug through granite , but some of the tunnel walls have been painted black to give the appearance of anthracite .
Upon their discovery , North Korea claimed that the tunnels were for coal mining ; however , no coal has been found in the tunnels , which are dug through granite . 
 Some of the tunnel walls have been painted black to give the appearance of anthracite .