In folklore, a will-o'-the-wisp, also called ignis fatuus, which means "foolish flame" in Latin, is a mysterious light seen in the air by travelers at night, especially over wetlands like bogs, swamps, or marshes.

In the United Kingdom, this light is known by other names, such as jack-o'-lantern, friar's lantern, and hinkypunk. It is said to trick or help travelers by looking like a flickering lamp. Similar lights appear in European folklore with names like feu follet in French, Irrlicht or Irrwisch in Germany, dwaallicht in the Netherlands, and fuoco fatuo in Italy. Other cultures worldwide also have names for this phenomenon, such as the Naga fireballs over the Mekong River in Thailand. In North America, it is called the Paulding Light in Michigan, the Spooklight in Missouri and Oklahoma, and the St. Louis Light in Saskatchewan. In Arab folklore, it is known as Abu Fanous.

In stories, will-o'-the-wisps are often described as ghosts, fairies, or spirits that show or hide paths. They are seen dancing or moving until someone notices them, at which point they fade or vanish. Scientists explain the light as natural events, such as bioluminescence or chemiluminescence, caused by chemical reactions from decaying organic matter, including phosphine, diphosphane, and methane.

Nomenclature

The term "will-o'-the-wisp" comes from the word "wisp," which means a bundle of sticks or paper used as a torch, and the name "Will," meaning "the owner of the torch." The term "jack-o'-lantern" ("Jack of the lantern") originally described a will-o'-the-wisp. In the United States, these lights are sometimes called "spook-lights," "ghost-lights," or "orbs" by people who study folklore.

The Latin name "ignis fatuus" is made up of "ignis," meaning "fire," and "fatuus," meaning "foolish" or "simple." This can be translated as "foolish fire" or "giddy flame." Although the term has Latin roots, it was not used by ancient Romans, and the name they used for will-o'-the-wisps is unknown. The term "ignis fatuus" does not appear in writings from the Middle Ages either. It was first recorded in the 16th century in Germany, where it was created by a German humanist as a translation of the German word "Irrlicht" ("wandering light" or "deceiving light"). This name came from German folklore, which described the phenomenon as a mischievous spirit of nature. The Latin term was used to make the German name seem more intellectual.

In German, the will-o'-the-wisp is also called "Irrwisch," where "Wisch" means "wisp," as seen in writings from the same period, such as those by Martin Luther.

In Irish, it is called "tine ghealáin" ("flashing fire") or "Seán na Gealaí" ("Jack of the Moon"), a name connected to the legend of Stingy Jack and the jack-o'-lantern.

The names "will-o'-the-wisp" and "jack-o'-lantern" appear in stories that explain the origins of the phenomenon, called "etiological folk-tales." These stories are found in many versions in Ireland, Scotland, England, Wales, Appalachia, and Newfoundland.

Folk beliefs link the phenomenon to names like "hob lantern" or "hobby lantern" (also called "Hob and his Lantern" or "hob-and-lanthorns"). In her book A Dictionary of Fairies, K. M. Briggs lists many other names for the same phenomenon. However, the location where it is seen, such as graveyards or bogs, often influences the name used. When seen in graveyards, it is sometimes called a "ghost candle" or "corpse candle."

Folklore

Mexico has similar stories. Folklore says the lights are caused by witches who change into these glowing lights. Another story says the lights show where gold or hidden treasures are buried, but only children can find them. These lights are called luces del dinero (money lights) or luces del tesoro (treasure lights).

In Massachusetts, a swampy area called the Bridgewater Triangle has stories about ghostly glowing balls of light. People have also seen these lights in modern times.

In Louisiana, the fifollet (or feu-follet) comes from French stories. The legend says the fifollet is a soul sent back from the dead to do a punishment for God, but instead, it harms people out of anger. It usually plays harmless tricks, but sometimes it harms children by sucking their blood. Some stories say it was the soul of a child who died before being baptized.

In Brazil, the will-o'-the-wisp is called Boi-tatá. It has many names, like Boitatá or Baitatá, and means "fiery serpent" in the Old Tupi language. It is said to have survived a big flood. A giant snake called boiguaçu left its cave after the flood and hunted animals and dead bodies, eating their eyes. The light from the eyes gave Boi-tatá its glowing eyes. It is not a dragon but a giant snake.

In Argentina and Uruguay, the will-o'-the-wisp is called luz mala (evil light). It is a major part of folklore in both countries and is feared, especially in rural areas. It appears as a very bright ball of light floating just above the ground.

In Paraguay, will-o'-the-wisps are believed to show where hidden treasures are buried. These treasures were hidden by the Jesuits after they were forced to leave in 1767 and during the Paraguayan War (1864–1870). People believe that a brief, moving flame that appears at night marks the exact spot where a treasure is hidden.

In Colombia, the will-o'-the-wisp is called la Bolefuego or Candileja. It is the ghost of a cruel grandmother who taught her grandchildren to be thieves and murderers. In the afterlife, her spirit is trapped in flames and wanders the world. In Trinidad and Tobago, a soucouyant is a "fireball witch" that takes the shape of a flame at night and enters homes to drink the blood of its victims.

In Bangladesh and West Bengal, a strange light called Aleya (or marsh ghost-light) appears over wetlands. Fishermen believe these lights are the ghosts of dead fishermen. Sometimes the lights confuse fishermen, but other times they help them avoid danger. In the Banni grasslands of India, a similar light called Chir batti dances on dark nights. Other ghost-light stories exist in India, such as Kollivay Pey in Tamil Nadu and Karnataka, Kuliyande Choote in Kerala, and stories from tribes in Northeast India. In Kashmir, a figure called Bramrachokh carries a pot of fire on its head.

In Japan, similar lights are called hitodama ("human soul") or hi no tama ("ball of flame"). These lights are linked to graveyards. Kitsune (fox spirits) are also connected to will-o'-the-wisps, as their offspring create kitsune-bi ("fox-fire"). These stories appear in books like Graphic World of Japanese Phantoms by Shigeru Mizuki.

In Korea, will-o'-the-wisps are called dokkebi bul ("goblin fire"). They are seen near rice paddies, old trees, or mountains and are believed to be mischievous, leading people astray or into danger.

In China, the earliest written record of will-o'-the-wisp is the character lín, which appears on ancient oracle bones. It shows a human-like figure surrounded by glowing dots. Over time, the character evolved to represent fire and is now part of the word lín (phosphorus), which is linked to scientific explanations of the phenomenon. A Chinese scholar named Shen Gua wrote about a glowing pearl seen in a marsh that was visible for over ten years.

In European folklore, will-o'-the-wisps are often linked to the spirits of unbaptized or stillborn children. In Germany, a light called Irrlicht was believed to be the soul of an unbaptized child, but it could be saved if its remains were buried near a church. In Sweden, the light was thought to be the soul of an unbaptized person trying to guide travelers to water for baptism.

In Denmark, Finland, and other northern countries, will-o'-the-wisps were believed to mark the location of hidden treasure. Some stories said that treasure could only be found when the light was present, and magical rituals or even a dead man's hand were needed to uncover it. In Finland, people believed the best time to search for treasure was in early autumn, as treasure was said to be hidden only during the summer solstice.

Scientific explanations

Scientists suggest that will-o'-the-wisp phenomena, also called ignis fatuus, happen when certain gases like phosphine (PH₃), diphosphane (P₂H₄), and methane (CH₄) react with oxygen in the air. These gases are created during the breakdown of organic matter, such as decaying plants or animals. When phosphine and diphosphane mix with air, they can catch fire easily, which might help ignite methane, a gas that is much more common. This reaction can create brief, glowing fires. Phosphine also produces phosphorus pentoxide, which turns into phosphoric acid when it touches water vapor. This might explain why some people describe a sticky, wet feeling near will-o'-the-wisps.

The idea that natural gases cause will-o'-the-wisps was first mentioned in 1596 by Ludwig Lavater. In 1776, Alessandro Volta proposed that methane gas from swamps might interact with natural electricity, like lightning, to create these lights. Joseph Priestley and Pierre Bertholon de Saint-Lazare later supported this idea in their scientific writings.

Some people doubted the marsh gas theory because they thought spontaneous combustion was unlikely, or because the lights sometimes did not feel warm. Others noticed that the lights would disappear when approached, and some described ball lightning as a type of will-o'-the-wisp. In 1891, John G. Owens wrote about these criticisms in his book Folk-Lore from Buffalo Valley.

The lights might disappear when approached because moving objects can stir the air, causing the gases to spread out. In 1832, Major Louis Blesson studied will-o'-the-wisps in a marsh in Germany. He saw glowing blue-purple flames and noticed that the lights would vanish when he tried to get close. After testing, he confirmed the lights were caused by burning gas. Charles Tomlinson later wrote about Blesson’s experiments in his book On Certain Low-Lying Meteors.

Blesson also saw differences in the flames in different places. In one marsh in Poland, the lights could be lit and put out but could not burn paper or wood. In another area, the flames left a sticky, oily film on paper instead of burning it. Blesson accidentally created will-o'-the-wisps in Germany when fireworks were launched near a swamp.

In 1936, The Scout's Book of Gadgets and Dodges described will-o'-the-wisps as mysterious lights that appear in swamps.

In 1980, British scientist Alan A. Mills tried to recreate will-o'-the-wisps in a lab by mixing phosphine and natural gas. He made a glowing cloud, but it was green and produced smoke, which did not match most eyewitness reports. In 2000, Mills suggested that will-o'-the-wisps might be "cold flames," which are glowing, low-heat fires that occur when certain compounds are heated just below the point of catching fire. Cold flames are usually blue and produce little heat. They can form from compounds like methane, alcohol, or wax, but it is unclear if they happen naturally.

In 2008, Italian scientists Luigi Garlaschelli and Paolo Boschetti repeated Mills’ experiments. They made a faint, cool light by mixing phosphine with air and nitrogen. Though the light was green, they noted that in low light, people might not notice the color. By changing gas concentrations and environmental conditions, they reduced smoke and odors. They also agreed with Mills that cold flames could explain some will-o'-the-wisp sightings.

In 1993, professors Derr and Persinger proposed that some will-o'-the-wisps might be caused by geologic activity. They suggested that tectonic forces could create electricity in rocks containing piezoelectric materials like quartz or silicon. This electricity might travel up through water vapor, creating lights that look like will-o'-the-wisps.

Some scientists think bioluminescent organisms, like certain fungi or insects, might also cause will-o'-the-wisp lights. For example, the honey fungus emits a faint glow during chemical reactions. Fireflies and other glowing creatures might also contribute to the illusion of moving lights. The white feathers of barn owls might reflect moonlight, making them look like will-o'-the-wisps.

Today, will-o'-the-wisp sightings are rare because many wetlands have been drained and turned into farmland. For example, the Fenlands of England, once a large swamp, are now mostly farmland.

Recent research has proposed a new explanation: tiny lightning sparks, called "microlightning," can form between methane bubbles rising in water. These sparks cause methane to react with oxygen, creating light and heat. This discovery provides a new way to explain will-o'-the-wisps without needing an outside source to start the fire.

Literature and popular culture

Two will-o'-the-wisps appear as characters in Johann Wolfgang von Goethe's short story "The Green Snake and the Beautiful Lily."