will-o'-the-wisp
A luminescent phenomenon caused by the oxidation of certain gases, such as methane (CH4) or phosphine (PH3), originating from the decomposition of organic matter. Its appearance is that of a small flame, emitting a pale light, whose color differs depending on the nature of the substances undergoing putrefaction. Will-o'-the-wisps are only visible at night or during twilight, when ambient luminosity is low, and travel preferentially through the air over swampy areas and at ground level in cemeteries. Their observation in these latter places has fueled the belief that they are the souls of the deceased, with many people still clinging to the irrational idea that this is a supernatural phenomenon.