will-o'-the-wisp, phenomenon known also as ignis fatuus and jack-o'-lantern. It is seen at night as a pale, flickering light over marshland. There is no generally accepted explanation for it; it may result from the spontaneous ignition of gases (e.g., methane) produced by the disintegration of dead plant or animal matter, or it may be a form of phosphorescence. The eerie lights have given rise to many superstitions.