Geologically Improbable Mountain is especially improbable considering that it has a total of SEVEN tunnel portals in it, one of them double tracked. Large scale tunnels are so large that you can stick your whole arm in them so that seeing into them is easy. Special tunnel liners were cast to add depth to the tunnels. They were cast over a hardware cloth frame using Hydrocal and paper towels. Under all that plaster and on top of the wire is lightly crumpled and stretched out aluminum foil. This adds a blasted rock texture to the inside of the tunnel liner. After being painted flat black, the light that enters through the portal shows up the folds created by the foil for a short distance before it all fades to black. The whole mountain is light sealed as much as possible and painted black on the inside to hide its hollow nature.
Supervising is my #2 son, Charlie.