Complex sentence: 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.
Split into simpler sentences: 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.