Usually, wall art is man-made… in the form of unsightly spray-painted vandalism or better quality graffiti. In days gone by the rage was all internal fresco decoration. It took masters like da Vinci and Michelangelo to perfect those techniques.
However, occasionally Mother Nature wants to paint her own picture for our pleasure. Then she scatters a few seeds about. On the odd occasion these seeds land on top of ruined walls and in so doing we are granted wall art with a difference.