Will-o’-the-wisps are pale, flickering lights that appear to hover over marshes, bogs, and swamps, typically at night. They have been reported across cultures for centuries, inspiring ghost stories and folklore on nearly every continent. The leading scientific explanation points to gases produced by rotting organic matter in waterlogged ground, though the phenomenon has proven stubbornly difficult to study or reproduce in a lab.
What People Have Seen
Descriptions of will-o’-the-wisps are remarkably consistent across time and geography. Witnesses report a soft, bluish or pale flame that appears to float just above the ground in marshy areas, sometimes bobbing or drifting as if carried by an invisible hand. The lights seem to retreat when approached, which gave rise to countless legends about spirits luring travelers into dangerous terrain. Stories of these sentient blue flames bouncing across marshes and bogs appear in cultures from Northern Europe to Australia.
The first recorded sighting comes from a 1340 text by the Welsh poet Dafydd ap Gwilym, who called the phenomenon “canwyll corff,” meaning corpse-candle, linking the lights to death and burial sites. The English-language term “ignis fatuus” (foolish fire) first appeared in 1563. Over the centuries, the lights accumulated a long list of regional names: jack-o’-lantern (simply “Jack of the lantern” in English), walking fire, corpse-candle, and many others. In one common version of the myth, the lights belong to a mischievous sprite called a puck, which uses them to lure people into ditches, bogs, and pools, then laughs at their misfortune before disappearing.
The Chemistry Behind Marsh Lights
The most widely accepted explanation involves gases released by decomposing organic matter in oxygen-starved wetland soil. When plant and animal material breaks down underwater, where oxygen can’t reach, microbes take over through a chain of fermentation reactions. These microbes gradually break complex organic molecules into simpler compounds. Eventually, once other chemical pathways are exhausted, the process produces methane, the same gas used in natural gas stoves.
Methane alone, however, doesn’t explain the lights. Its auto-ignition temperature is 537°C, far too high to catch fire on a cool night in a swamp. The key ingredient is a companion gas: diphosphane, a compound made of phosphorus and hydrogen that forms alongside methane during the same anaerobic decay. Unlike methane, diphosphane ignites spontaneously at room temperature when it contacts air. As bubbles of these mixed gases rise through standing water and break at the surface, the diphosphane can ignite on its own and act as a spark for the methane around it. The result is a brief, ghostly flame hovering just above the waterline.
A related compound, phosphine, is also produced in marshes and is responsible for the rotting-fish smell common around stagnant water. Phosphine can burn too, but only at concentrations above 1.8% and temperatures around 38°C. Diphosphane is the more likely culprit for spontaneous ignition in cooler conditions.
Volta and the Discovery of Marsh Gas
The scientific investigation of will-o’-the-wisps started almost by accident. In November 1776, Alessandro Volta was boating on Lake Maggiore in Italy when he poked the muddy bottom near some reeds with a stick. He watched bubbles rise to the surface and collected the gas, discovering it was flammable. He called it “inflammable air from marshlands.” It was methane.
Volta’s curiosity led him to build a device later called “Volta’s Pistol,” which could ignite a mixture of the gas in a closed chamber. It wasn’t much use as a weapon, but it became an early tool for measuring gas composition and is sometimes considered a precursor to the internal combustion engine. His work established that swamp mud actively produces flammable gas, giving future scientists a starting point for explaining the floating lights that had mystified people for centuries.
Why the Lights Are So Hard to Study
Despite a plausible chemical explanation, no one has definitively proven it by capturing and analyzing a will-o’-the-wisp in the wild. A paper in the Philosophical Transactions of the Royal Society A put it bluntly: “the only certainty is that no modern experimental analysis has been carried out on this phenomenon.” The biggest obstacle is that the lights have become extraordinarily rare. There are essentially no confirmed modern sightings, and researchers have speculated about why.
One likely factor is the dramatic loss of wetlands over the past two centuries. Marshes and peat bogs have been drained for agriculture and development across Europe and North America, eliminating the very ecosystems that would produce the right mix of gases. Light pollution may also play a role. A faint flame that would have been visible on a pitch-dark medieval night could easily go unnoticed near a modern road or town.
Italian chemist Luigi Garlaschelli has come closest to reproducing the effect in a laboratory, experimenting with compounds released by rotting plants and animals until he got a gas mixture to ignite on its own. But as other scientists have pointed out, his experiments were conducted in water, while actual will-o’-the-wisps are flames that hover in air above the marsh surface. The gap between lab conditions and the real phenomenon remains open.
Other Possible Explanations
Not everyone is satisfied with the marsh-gas theory, and a few alternative explanations have been proposed. One involves bioluminescent fungi. The mycelium of the marsh honey fungus is known to glow faintly, and it has been documented at a small number of sites in Great Britain. However, fungal bioluminescence looks quite different from the flickering, mobile flames described in historical accounts. It produces a steady, dim glow close to the ground rather than a dancing light that appears to move through the air.
Some researchers have suggested that certain sightings could be explained by barn owls, whose pale feathers can appear to glow when seen at a distance in low light, or by unusual atmospheric conditions that refract distant light sources. Ball lightning, another poorly understood phenomenon, has also been proposed as a possible overlap. None of these alternatives account for the full range of historical descriptions, particularly the association with marshes, the bluish color, and the apparent responsiveness to movement.
The Phenomenon in Culture
Whatever their physical cause, will-o’-the-wisps left a deep mark on language and storytelling. The term “ignis fatuus” became a metaphor for any deceptive goal or false hope, something that lures you forward but can never be reached. Shakespeare’s Puck in “A Midsummer Night’s Dream” draws on the same folklore tradition of a trickster spirit using light to mislead travelers. The jack-o’-lantern, before it became a carved pumpkin, referred to these mysterious marsh lights.
The phenomenon sits at an interesting intersection of science and mythology. The chemistry is plausible, the historical accounts are abundant, but the direct evidence connecting one to the other remains frustratingly thin. Modern instruments capable of analyzing the gas composition and spectral signature of a will-o’-the-wisp are ready and waiting. The lights just need to cooperate.