In 2016, a spooky white figure with wide, unblinking eyes was spotted wandering a Green Bay, Wisconsin parking lot at two in the morning.

Residents posted about their sightings on Facebook and made calls to the police.

But the strange figure continued appearing that night in a local park and under an overpass.

Then, it suddenly disappeared just as mysteriously.

A week later, a similar being appeared in South Carolina, beckoning to residents from the woods.

Over the next two months, dozens of these mysterious figures would pop up all across the U.S. and Canada, along with sightings in 16 other countries around the world.

Some would attempt to approach children to frighten them.

Most appeared silently, watching, but all of them were dressed as clowns.

Clowns are known for their humorous antics, balloon animals, and juggling to amuse audiences, whether they're teaching us how to tell time on Saturday morning TV or selling us french fries.

So when did these silly, funny staples of our childhoods become monsters?

[adventurous music] I'm Dr. Emily Zarka, and this is "Monstrum."

The history of clowns spans centuries.

We associate them with bright-colored clothing, oddly-shaped hats, oversized shoes, and painted faces.

Our current image of clowns started taking shape around the mid-19th century when an English comedian and actor named Joseph Grimaldi began performing in a polka-dotted costume, pointy, triangular wig, ruffled collar, and red-and-white face paint.

[gentle music] But since the 1980s, clowns have taken a decidedly dark turn.

Depictions of clowns have taken these visual traits from kooky to creepy by playing up features like pale skin, big eyes, and a wide, unnerving smile.

[clown laughs menacingly] These creepy clowns appear like the silly characters of our childhoods, just slightly off.

So why the shift?

Even before killer clowns were all over our screens, clown characters have occupied an unusual place in the social order.

Some of the most ancient versions were fairly problematic.

In places like Ancient Egypt and Rome stigmatized, marginalized people, many enslaved, were treated as entertainment at parties and events for the wealthy.

Ancient Rome also had mime-type characters and ones who performed unusual body stunts.

Beginning at least as early as the 13th century, Chinese Opera featured clown characters called Chou who were often the only ones on stage allowed to improvise lines, and Renaissance-era court jesters in Europe were kept in noble circles as entertainment and were permitted to satirize royalty and speak truths without fearing punishment.

Clowns throughout the world began as trickster characters who had a pass to flout social conventions and behave in transgressive ways.

By the mid 1800s, clowns began to filter from their high-class entertainment origins into popular culture.

Joseph Grimaldi became a celebrity in the English theater scene.

Meanwhile, in the U.S., the Ringling Brothers and Barnum & Bailey set up circus shows featuring clowns performing slapstick comedy routines.

As clowns rose in popularity, they retained their roguish spirit.

It wasn't until literary works like Charles Dickens' "Pickwick Papers," Edgar Allan Poe's "Hop-Frog" and Ruggero Leoncavallo's opera, "Pagliacci" that the clown persona took on a much darker tone.

These depictions were some of the first to look at the man behind the mask depicting clowns that, underneath their trickster silliness, have sinister or even murderous intentions.

[Pagliacci singing in Italian] But by the mid-20th century, American clowns got a kid-friendly makeover featuring more friendly figures like "Howdy Doody's" Clarabell the Clown, Bozo the Clown, and Ronald McDonald.

This reflected the broad optimism and prosperity of the post-war era.

But this wholesome imagining of the clown didn't last long.

In 1979, serial killer John Wayne Gacy, who worked as a party clown as a day job, was arrested.

At the same time, the Satanic Panic of the 1980s was picking up speed.

This moral frenzy based on conspiracy theories that evil cultists were harming children made many people anxious about who they could trust.

Against this cultural backdrop, creepy clowns started to become a pop culture staple, often using their kid-friendly cultural status to lure children to their deaths.

Their exaggerated facial features pushed clowns into the uncanny valley, a theorized point where an object or character appears almost like a real person without looking fully human.

It's that tiny bit of dissonance which fills actual human viewers with unease.

Film depictions use this creepy dissonance to create frightening characters.

"Poltergeist," a movie in which a possessed clown toy attacks a young boy in his bedroom, was a box office hit and "Batman" starred horror movie stapled Jack Nicholson playing Joker as a villainous clown.

Even children's media turned clowns exclusively scary.

Who else has nightmares about the evil firefighter clown from "Brave Little Toaster?"

'Cause I hate it.

Perhaps most famously, Stephen King's 1986 novel, "It" features Pennywise, an evil clown that stalks the suburbs looking for children to frighten and kill.

The book became so successful that it was adapted as a TV series by the end of the decade, solidifying the clown as not just a man in face paint, but a monster that could transform into whatever you fear most.

The 1988 cult sci-fi horror-comedy "Killer Klowns from Outer Space" turned this newly-minted archetype on its head with a more playful take on the killer clown featuring murderous aliens disguised as clowns who trapped their unsuspecting victims in cotton candy cocoons.

The film's decision to steer the killer clown archetype back towards its whimsical origins shows how prevalent the creepy clown phenomenon had become.

The last decade has brought in another wave of creepy clowns in pop culture.

Horrifying harlequins cropped up in "American Horror Stories," "Supernatural" and "Zombieland."

(character) Time to nut up or shut up.

[clown growls] "It," "The Joker," and "Poltergeist" all got darker, grittier reboots, but then, the clowns we love to fear on screen got a lot less fictional.

In the fall of 2016, that mysterious clown appeared in Wisconsin.

It didn't hurt anyone.

It just broke social norms, as clowns have always done.

By creeping around at odd hours and remaining silent when spoken to, this clown didn't act quite human.

The prankster eventually revealed that he was trying to drum up publicity for an independent horror film, and it worked.

Photos and stories about the clown sightings went viral on Facebook, inspiring a wave of copycat clowns in more than 20 states over the next two months.

If you remember the late summer of 2016, you know that it was a volatile and politically divisive time.

It's possible that these tricksters wanted to let off some of the tension that was building in their communities or to direct anxieties at a common fictional enemy.

But they successfully tapped into the public's fear of these uncanny-valley creatures, enough that some school districts and municipalities warned against, or even banned, wearing clown costumes at Halloween for being symbols of terror.

Police in Utah even received calls about whether citizens were allowed to shoot clowns on sight.

And when rumors started circulating about a clown on Penn State's campus that October, more than 500 students took to the streets in a so-called clown riot to hunt the jester down.

And yet, just like in the '80s, clowns were more of a psychological threat than a physical one.

With their exaggerated smiles, it's difficult to read what clowns are really thinking or feeling underneath all that face paint and their reputation for mischievous behavior makes it hard for us to predict what they might do next.

And all of that uncertainty makes us uneasy.

It's no wonder that killer clowns tend to emerge in pop culture at times of heightened political tension or distrust in the social contract.

When we're all wondering whether we can trust the people around us.

Clowns may be a symbol of human duality, one that asks us to consider if we all wear a little bit of a costume when we present ourselves to the world.

Ruggero Leoncavallo's opera "Pavliacci."

-(Producer) "Pagliacci."

-"Pagliacci."

There's no V?

Wait there's... -Oh!

-"Pavliacci?"

I didn't even pick that up.

Did I say it right, though?

You're like, "Uh..."