Ignis Fatuus (Fool's Fire, Foolish Fire, etc.), is a phenomena reported under different names in almost every single culture.
Appearance: An eerie ball of light that is seen to float or dance through the air around swamps, bogs, woods, and other wild locations.
Lore: In folklore, such lights are attributed to numerous supernatural beings including but not limited to, ghosts, fairies, witches, vampires, demons, and shape-shifters.
Scientific Explanation: The majority of sightings of Ignis Fatuus can be attributed to "swamp gas", i.e. methane and other gases naturally produced in such locations which under the right conditions can combust, producing brief lights.
Trivia: J. Allen Hynek, proposed the explanation of "swamp gas" for UFO sightings occurring in Michigan in 1966. Since then, Skeptics have repeatedly used "swamp gas" as a possible explanation for various UFO sightings.