Per wikipedia: "In folklore, a will-o'-the-wisp, will-o'-wisp, or ignis fatuus (Latin for 'foolish flame'), is an atmospheric ghost light seen by travellers at night, especially over bogs, swamps or marshes. The phenomenon is known in much of European folklore by a variety of names, including jack-o'-lantern, friar's lantern, and hinkypunk, and is said to mislead travellers by resembling a flickering lamp or lantern. (...) In folklore wills-o'-the-wisp are typically attributed as ghosts, fairies, or elemental spirits. Modern science explains the light aspect as natural phenomena such as bioluminescence or chemiluminescence, caused (...) by organic decay. "