I wonder if Bob Ross ever imagined some of his 'Liquid clear' techniques for global shading would be used by graffiti artists.
Ancient Egyptian graffiti exists in the Great Pyramid as well as the valley of the kings. They range from just names to sexual acts on the intended honored. Roman graffiti is a subject of its own. Some folk tunes are the musical graffiti of its time. - jus ramblin

Mostly I am impressed by the cliffside painting discovery