- [Emily] Ethereal and fleeting, mesmerizing and elusive, these floating blue flames have been a mainstay in European folklore since at least the 14th century.

You may have heard of the name will-o'-the-wisp, but do you know what it is?

Stories of sentient blue flames floating, and bouncing across marshes and bogs, permeate cultures from Northern Europe to Australia.

While some stories have faded into ephemera, just as the floating blue flames that inspired them, these elusive creatures remain mainstays of myth, and legend in pockets across the globe.

So what are they?

Fairies, fireballs, ghosts, atmospheric phenomena?

Here's the haunting history of the will-o'-the-wisp.

(upbeat instrumental music) I'm Dr. Emily Zarka.

And this is "Monstrum."

The will-o'-the-wisp are most frequently described as small moving blue flames that hover a few feet off the ground, and do not flicker.

Cold or cool to the touch, if you're brave or reckless enough to touch one, they can be found in marshlands, bogs and swamps.

All treacherous places to wander even without the alluring glow of the will-o'-the-wisp leading you deeper into danger.

Follow the will-o'-the-wisp's seductive dance, and you may find hidden treasure or a watery grave.

Stories of these elusive, and inscrutable flames vary.

Some say spotting the floating flame is an indication that someone is blessed with the power of foresight, but more often folklore, and literature paints them as impish tricksters, restless spirits or malevolent beings.

In Sweden, will-o'-the-wisps are ghosts with lanterns fated to wander aimlessly for removing their neighbor's boundary markers.

In English tradition, the blue flames are lights carried by elves.

And in Australia, the light may approach someone, but anyone who tries to catch it will disappear.

Other tales say the lights are crafted by fairies or even a fire species on its own.

In Belgium and the Netherlands, they are the lost spirits of unbaptized children.

While in other places, the floating flames are said to be the wandering souls of unhappy women, unrighteous men, priests who have broken their vow of chastity, or even those souls who escaped purgatory.

In fact, the ethereal-like leads its most enduring name will-o'-the-wisp or will-with-a-wisp from the Saxon word wile for fraud, trick, or deceit and the Swedish word wisp, meaning a small lit bundle of tinder.

Variations include will-with-the-wisp, willy wisp, and willow o' the wykes.

Essentially, the name hints that the light comes from some trickster spirit holding a flame aloft.

Further north in Norwegian folklore, the will-o'-the-wisp is called Hoberdy's Lantern, Hobany's Lantern or even Hob and his lantern.

Hob may come from the Norwegian hoppe, meaning mare, or the old Norse word hoppa, which translates to leap or hop, which might not be as obvious a name as a deceitful bundle of tinder until you know that the will-o'-the-wisp movement often resembles the cantering motion of a horse.

Yet another moniker for the floating lights, Jackie lantern or jack-o'-lantern.

Yes, that's right.

The carved pumpkins that adorn many a stoop or doorstep on Halloween actually borrow their name from this mysterious light.

Jack-o'-lantern is of English language origin, and simply means Jack of the lantern.

So when did we start hearing about these creatures?

Well, the first recorded siting of will-o'-the-wisp is described in a 1340 text penned by Welsh poet Dafydd Ap Gwilym.

He wrote that, "In every hollow live a hundred wrymouthed wisps."

He names the phenomenon Canwyll Corff, Welsh for corpse-candle, thereby introducing what will become another common name for the supernatural blue flames, and associating the lights with death, and burial sites.

The first appearance of the lights in the English language occurs in 1563 under the name Ignis Fatuus.

Here they are described as, "Foolish fire that hurteth not."

In another variation of the will-o'-the-wisp myth, the ghost lights are attributed to a mischievous sprite called a puck, which uses the light to lure humans to fall into ditches, bogs, and pools, and then laughs with their predicament before fleeing.

Sometimes referred to in this form as walking fire, it can also appear as a horse, bull, or eagle, but it is always a trickster.

Shakespeare adopts the name Puck for the character of a shape-shifting sprite in "A Midsummer Night's Dream."

First performed around 1596.

This Puck turns into a phosphorescent glow, and hovers over the marshes at night to trick travelers.

In "Henry IV."

He uses the name ignis fatuus to refer to a ball of wildfire.

The will-o'-the-wisp even appears in John Milton's famous epic poem, "Paradise Lost."

As a malevolent distraction, "A wandering fire compact of unctuous vapor, which the night condenses, and the cold invirons round, kindled through agitation to a flame, which oft, they say, some evil spirit attends.

Hovering and blazing with delusive light.

Misleads the amazed night-wanderer from his way, to bogs and mires, and oft through pond or poole."

Like many other references in literature, and folklore, Milton's will-o'-the-wisp is associated with danger, bad news and even death.

But over time, the elusive blue flame also became a literary metaphor for anything that is just out of reach, a delusion one may chase endlessly, but unsuccessfully.

Will-o'-the-wisp sightings are reported across the globe, and the frequency of their marshland appearances points to a very real origin.

The stories may be made up, but the lights themselves are not.

There's scientific truth to these reports.

Theories explaining the will-o'-the-wisp phenomenon circulated in scientific communities for 100s of years.

In fact, in 1704 Isaac Newton wrote in "Opticks."

Of the ignis fatuus as, "Vapors shining without heat."

The vague statement unsurprisingly sparked vigorous debate in the academic communities of the 18th and 19th centuries.

English folklorist and historian, Jabez Allies traveled over Europe collecting stories, and evidence of antiquity.

And in 1839 he got lucky with his own firsthand experience catching a glimpse of the mysterious flame.

Allies claims that while traveling near Worcester in England, he saw the will-o'-the-wisp.

He described the light as very clear and strong, unwavering and much bluer in color than a candle.

He wrote, "Sometimes it was only like a flash in the pan on the ground; "at other times it rose up several feet and fell to the earth and became extinguished; and many times it proceeded horizontally from fifty to one hundred yards with an undulating motion, like the flight of the green woodpecker, and about as rapid; and once or twice it proceeded with considerable rapidity in a straight line upon or close to the ground."

Allies reports that the two nights in a row he saw the phenomenon, there were only minimal clouds, and bright starlight.

Both evenings were rather warm and had no fog.

He speculates that the will-o'-the-wisps appear on nights after a rainfall in the winter season.

Was Allies onto something?

What else could explain these odd flames?

Fireflies?

Too small, and they glow with a yellow or green flickering light.

Glowworms?

The females do produce a bright light, but don't fly.

Male glowworms fly, but don't glow.

St. Elmo's fire, perhaps?

The plasma-creating weather phenomenon does produce a blue glow, though it requires a strong electrical field in the atmosphere, and an encounter with a conductor like the mast of a ship or the wing of a plane, which doesn't appear in reported will-o'-the-wisp sightings.

What about ball lightning?

Spherical, glowing, and capable of lasting for longer than a brief flash, yes, but only during thunderstorms with ranges in color, and usually accompanied by a hissing sound, and a distinct odor.

Will-o'-the-wisp aren't associated with any smell, and only seem to appear a day or more after rainfall or storm.

Perhaps then, dare I say it, luminous owls?

An unusual occurrence in which the feathers of the bird are contaminated by a fungus whose metabolic process produces a glow when the owl is in the dark.

But will-o'-the-wisp move more slowly than owls typically hunt, and don't tend to cover much ground.

The most likely explanation, spontaneous combustion of marsh or swamp gas, a rationale proposed at least as early as 1729 by W. Durham who, after observing the phenomenon over half a decade, directly refuted the hypothesis that the lights were caused by glowworms flying together.

Given the frequently reported occurrence of the will of the will-o'-the-wisp on marshlands, swamps and bogs, swamp gas seems pretty plausible.

Made up of about 60% methane as well as other components like carbon dioxide, swamp gas is the by-product of decomposing vegetation in areas like swamps marshes and even landfills.

And in some rare cases, if there is enough heat, and oxygen produced by the metabolic process, the gas can spontaneously combust, and produce fire.

And nothing just happens to burn with a blue flame and a yellowish glow.

Will the mystery of the will-o'-the-wisp ever truly be solved?

Even today many people swear the will-o'-the-wisp is a supernatural or paranormal experience.

Interestingly, there have been no modern encounters with the will-o'-the-wisp, and therefore no new data to add to any potential debunking theories.

Perhaps there are no more will-o'-the-wisp because we have drained, and demolished so many marshes and lowlands.

Maybe we'll never know what caused the glowing blue flames spotted so many years ago.

And I kind of liked that.

Glow on mysterious fiery orbs.

Glow on.