Marsh light, historical artwork. Also known as a will o' the wisp, ignis fatuus, and corpse candle, a marsh light is a pale light seen in marshes and bogs. It is believed to be caused by spontaneous combustion of methane, or other flammable gases, produced by decaying organic matter. Legends tell that unwary travellers (such as at right) can be lured off safe paths by these lights, and drown in the marsh. 19th century engraving.