Simplify the following text for a Grade 6 native English-speaking person.
Original: 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 .
Simplified: All tunnels were dug in a rock called granite . Some of the tunnel walls have been painted black to give the appearance of anthracite .