There is an obvious visual metaphor here reflected in th title. These are examples though of “Wet Wood”, a colouless slime that exudes from cracks in a tree’s bark and which blackens upon contact with air. This phenomenon is caused by a type of microbe called a methanogen, that grows in the absence of air and which produces amongst other things methane. If the internal pressure of the gas is high enough, it can be ignited to produce a jet of flame. Another signature of a sublime and invisible world.