Will-o'-the-wisp

There is no single origin story; names and etiologies vary by region. Folkloric explanations include wandering lights of nature-spirits, souls of the dead (corpse/ghost candles), witches transformed into lights, returned souls performing penance (Louisiana fifollet), markers of buried treasure (Mexico, Paraguay), or serpent-like fiery beings (Brazilian Boi-tatá, from Old Tupi mboî tatá). The learned Latin term ignis fatuus ('foolish fire') appears in early modern literature (16th century) as a translation of German folk names such as Irrlicht rather than as an ancient Roman concept. Modern scientific interpretations (chemiluminescence from gases produced by organic decay) are separate, post-folklore explanations noted alongside traditional accounts.

High-level common features recorded across sources: a small, flickering, dancing or floating ball or lamp-like light seen at night, often hovering low over ground, especially marshes, bogs, or graveyards. Regional variants supply distinct imagery: the Brazilian Boi-tatá described with great fiery eyes and serpentine form (mboî tatá), the Argentine/Uruguayan luz mala as an extremely shiny ball of light floating a few inches above the ground, and graveyard variants referred to as ghost or corpse candles appearing among graves.

Folkloric behaviors include wandering or dancing motion and the tendency to resemble lamps or lanterns that may lead travellers astray or, alternately, guide them; when followed or closely observed the lights commonly seem to fade or disappear. Some local traditions attribute harmful actions to particular variants: the Louisiana fifollet is described as a returned soul that mostly mischiefs but sometimes attacks or sucks the blood of children; the Boi-tatá is said to eat the eyes of animals and corpses and to possess great fiery gaze at night. Other traditions treat the lights as indicators of buried treasure or as omens. Modern scientific accounts interpret some sightings as natural chemiluminescent phenomena caused by oxidation of gases (phosphine, diphosphane, methane) from organic decay.