Original subtitles

[intense music]

- Tonight, the search for the Yeti,

a creature of myth allegedly lurking

amongst the highest peaks

of the mountains of Eastern Asia.

[intense music continues]

- Tales of the Yeti,

this mythical beast from the high mountains

of Asia and Siberia,

have existed for thousands of years.

[intense music continues]

- Traditional beliefs that the Yeti

is actually protecting the sacred mountain peaks

and that if humans encroach on those spaces,

they get killed on sight.

- If the Yeti does exist,

is it a living being or something else entirely?

[intense music continues]

- This is a species that should have gone extinct

thousands of years ago.

- No normal animal could survive

at those altitudes indefinitely.

- Now, we'll explore the top theories

surrounding this legendary creature.

- This is a tooth that comes from a creature

that was kind of like an orangutan,

except that this one is around 10 feet tall

and potentially weighs around 1,200 pounds.

- It's hard to say 2,000 years

of eyewitness accounts is all just mistaken identity.

So really what is happening here?

- At that high an altitude,

maybe people don't see what they think they're seeing.

- What exactly is the Yeti,

and could it really exist?

[intense music continues]

[intense music]

April 9th, 2019, the Himalayas.

An Indian army expedition

is on a remote mountain pass

on the border of Nepal and China

when they make a bizarre discovery,

massive footprints in the snow

that don't match any known animal on earth.

- The footprints that they find don't look human

in any way, shape or form.

The footprints measure about 32 inches long

by 15 inches wide,

which is significantly beyond

what any normal human footprint should look like.

- They do what any of us would do nowadays,

which is they tweet it.

They take pictures.

They post them online

from the official Indian Army Twitter account.

- They send a tweet to the world saying,

we have found the Yeti,

and the scientific world

and the crypto zoological world explodes.

[intense music continues]

Tales of the Yeti,

this mythical beast from the high mountains

of Asia and Siberia,

have existed for thousands of years,

and what they describe it as

is a large, towering, fanged, bipedal creature

that is covered from toe to head in hair,

and it has the features of both an ape and a human.

[intense music continues]

- [Laurence] The first recorded sighting

of the Yeti occurs in the fourth century BCE

- In 326 BCE,

as Alexander is attempting to conquer the Himalayas,

he hears these stories about this wild man of the snows,

and so he sends members of his army out to go

and see if they can find this beast,

but to no avail.

No monster or no wild man is ever caught.

[intense music continues]

[horns blaring]

- The origins of the Yeti,

where it actually begins, is unknown,

but we can go back to seventh century CE

where if you look at Tibetan folklore,

they describe a wild, hairy man of the mountains,

originally known as the glacier beast.

[intense music continues]

And if you look at Tibetan religious beliefs,

they see the Yeti as actually a god of the forest

and all the animals that live within it.

- There's not consistent agreement

about the nature of this creature.

Everyone pretty much agrees that it exists.

[intense music continues]

Some cultures feature him

as feral, and aggressive, and dangerous.

And others describe it more

as shy or even mischievous.

[intense music]

There are ideas and traditional beliefs

that the Yeti is a spiritual protector,

that it's actually protecting the sacred mountain peaks,

and that if humans encroach on those spaces,

they get killed on sight.

[flags flapping]

[intense music]

- [Laurence] The Yeti first captures

the modern world's imagination

at the turn of the 20th century.

- In 1899, Scottish explorer Laurence Waddell

publishes his book, "Among the Himalayas,"

and in it he talks about an excursion that he's going on

while in the Himalayan Mountains.

- He recounts a story of being at 17,000 feet

and coming across a series of tracks,

and he looks to his guides and is like, "What is this?"

They say, "This is the wild, hairy man of the eternal snows."

- In 1921, the explorer Charles Howard-Bury,

he's up around 20,000 feet,

and he and his guides also see this same thing,

these giant footprints in the snow.

So he pulls out his binoculars,

and is scanning the horizon,

and he actually sees something.

He sees off in the distance against the snow,

this giant lumbering creature.

Now, it's far away.

He can't see a lot of details of it,

but it's really clearly there,

stark against the snow.

When he asks his guides,

"Okay, so what am I seeing here?"

they also have an answer for him.

"The wild man of the snows."

They call it the Meetoh Kangmi.

[intense music]

- When Howard-Bury is later interviewed

by a journalist about his experience in the Himalayas,

he mentions Meetoh Kangmi.

The reporter mistakenly substitutes the word Meetoh,

which means wild, with mech, which is filthy.

So little journalistic embellishment.

Little playing around with some language.

And because of this,

the Yeti now gains a brand new title

and becomes known henceforth as The Abominable Snowman.

[intense music continues]

- The idea that there is

some undiscovered, giant, bipedal creature

living in the mountains of Asia and Siberia,

well, that just stokes the fires

of the scientific community,

and it basically begins this wonderful charge

into the mountains to discover this.

[intense music continues]

And who's gonna discover it first?

- In 1951, you've got two British explorers,

Eric Shipton and Dr. Michael Ward,

who make their way to the Himalayan Mountains,

and they are again, a part of an exploration.

As they're ascending these mountains,

they also come across these footprints in the snow.

[intense music]

- The footprints are really intriguing

because it's a very low, bulbous big toe,

almost more like a thumb than a toe,

and then three or four additional toes

that seem kind of hooked,

spaced out differently.

Unmistakably a foot, and seemingly not human at all.

- In their letters home, Shipton and Ward

are describing these footprints,

and they're stating that there's no possible way

that any animal or any human

could have created these footprints in the Himalayas.

So they conclude

that these footprints do indeed belong to the Yeti.

[intense music continues]

- About a decade later, Dr. Ward in 1961

is at a base camp at about 19,000 feet,

and he meets this Nepalese man.

- He shows up in just a wool coat

with no shoes and no gloves.

[intense music continues]

Now, he spends the next 14 days without a tent,

and the temperatures were in the single digits,

and then at night getting even colder.

And the most amazing part about it

is that he didn't get frostbite,

- But that's not the only thing.

His feet do not look like a typical human foot might look.

The skin is thick.

The toes are misshapen and bulbous.

[intense music continues]

- Ward is starting to think, well,

if he's not really affected by the snow,

and he's also walking around barefoot,

is it possible he's creating

these odd footprints in the snow?

[intense music continues]

Over time, Ward has spoken

to a lot of Himalayan residents of the area,

and he's come to realize that it's not just one man

that's walking around barefoot,

but a number of people living in this Himalayan region

are doing the exact same thing,

walking around barefoot.

[mysterious music]

- So later on,

Dr. Ward really delves into this theory.

He actually puts out this theory

of cold-induced vasodilation

so that instead of the blood vessels in the feet

and the extremities immediately constricting

and causing frostbite as they typically would,

they dilate and constrict, dilate and constrict,

which allows people to be barefooted in the snow

in these temperatures for extended periods of time.

[mysterious music continues]

- [Laurence] Ward also proposes that these foot deformities

are more common in certain areas of the Himalayas.

[mysterious music continues]

- According to Ward's theory,

we have these remote villages, no doctors, no clinics,

and these genetic defects.

And so it would make sense that these people

that are born with these things

that can't get operated on

just have to deal with it.

In these extremely remote regions,

the villages are very tiny,

and so with just tens of people,

you would imagine that these genetic defects

with a small gene pool and possibly some inbreeding

would be passed on from generation to generation.

- And that would help explain why these sightings

have been so consistently reported over time.

It's not one person.

It's an entire lineage of people

with feet shaped this way

walking barefoot over the snow.

- Additionally, people in the Himalayan region

look at wearing shoes as incredibly formal,

which is something that they don't necessarily want to do.

On top of that, if you're somebody that has a foot

with all of these deformities on it,

wearing shoes is gonna be incredibly uncomfortable.

- When we put these things together,

the shape of people's feet,

the fact that it would be hereditary,

the fact that they have this almost superpower

to walk barefoot over the snow,

we get a really plausible explanation

for photos like Eric Shipton's.

[intense music]

- Ward's theory offers a compelling answer

to what could be making these tracks,

but what about the large,

hulking, bipedal creatures that people are seeing?

It does not explain that at all.

[intense music continues]

[intense music]

- [Laurence] The Yeti is commonly described

as a tall, menacing creature

covered head to toe and shaggy hair,

but it's possible many witnesses over the generations

have fallen prey to a case of mistaken identity.

- When it comes to eyewitness accounts of the Yeti,

most of these eyewitnesses spy the Yeti

from a pretty far distance,

or if they do come across any sort of evidence,

like stray hairs for example,

these are stray hairs that could potentially belong

to any number of creature.

- There's a really great example of it from 1832,

this explorer B.H. Hodgson.

He's in the Himalayas.

He has a bunch of native guides with him,

and they all report seeing this kind of creature,

tall, hairy, bipedal, off in the distance.

[suspenseful music]

- And as they approached the creature,

it shot off into the woods.

And he was like, "What the heck could that possibly be?

It has to be the Yeti."

- And so after he published this account,

he actually backtracked

and said, "Must have been an orangutan."

[suspenseful music continues]

You've gotta wonder why would an orangutan,

who typically lives in Sumatra

3,500 miles to the south in a tropical jungle

be doing up in the Himalayas?

The only feasible excuse would be a traveling circus.

But why would a circus be trying to summit Everest?

- It's possible that he picked an orangutan

maybe because that was the only type of creature

known to him at the time that would resemble

what he potentially saw in the snows during his excursion.

- There's really a lack of hard evidence,

so researchers who want to take this question seriously

know that they need something more.

They need something like DNA.

- In 2012, a professor at Oxford named Bryan Sykes

sends out this public appeal

to everyone around the world, basically,

send me your Yeti samples.

And he gets like 57 samples.

[suspenseful music continues]

- About half of them, 30 of them are viable,

and he begins testing them.

And he finds what we might expect,

a large number of very normal animals,

horses, and dogs, and bears.

- [Laurence] But there are two samples

that really stand out.

[intense music]

- One of those samples comes from a hunter

that shot this giant beast 40 years prior in the Himalayas,

and the other sample

comes from this high-altitude area in Bhutan.

- Both of these samples, upon analysis,

reveal that they closely match

a known species, Ursus maritimus, the polar bear.

[intense music continues]

Sykes, speculating on this, suggests

that maybe what we're seeing here

is some sort of descendant

of an earlier polar bear species.

[intense music continues]

- Now, there are some researchers who speculate

that perhaps the polar bear

that is being found in these genetic samples

might actually be a subspecies

of polar bear called Ursus maritimus tyrannus.

Ursus tyrannus was a monster of a species.

On its hind legs, it stood as tall as 12 feet

and could weigh over a ton.

Now, Ursus tyrannus went extinct about 40,000 years ago,

but some speculate that it is possible

that maybe Ursus tyrannus bred

with other local bears in the region

over a number of years

and created a new species.

- So polar bears actually have a lot of characteristics

that make them a pretty good contender for the Yeti.

For one, they are perfectly adapted

to incredibly cold climates.

So these are creatures that are meant to live in conditions

that human beings cannot survive within,

and they're also really solitary.

- They will usually roam for hundreds of miles

by themselves in search of food.

So it is possible that if you have this solitary figure

roaming these snowy environs on its own,

seemingly unfazed by anything,

it is possible that that particular species of bear

could be mistaken for the Yeti from a distance.

- The fact that bears walk around on four feet

would have people believe,

well, there's no way to confuse that with a Yeti,

but if you think about bear prints,

the front paw actually combines with that back paw print,

which would form a large single print,

which could look like a bipedal Yeti footprint.

[intense music continues]

- [Laurence] But when the two key samples from the study

are subsequently retested, there's an issue.

- Two separate molecular biologists

take a look at Sykes' as results,

and they contest that the samples

that he was looking at to extract DNA from

were actually damaged.

[intense music continues]

- DNA is a fragile thing.

And so if these samples had not been kept

at the right temperature

with the right humidity,

they break down very quickly,

and after further examination,

the molecular biologists believe

that these DNA samples actually match more closely

the Himalayan brown bear,

which is much smaller than a polar bear.

- But Sykes himself, he stands by his interpretation

of this possible descendant of Ursus tyrannus.

Either way, a descendant of Ursus tyrannus

or a local bear,

we still have a plausible explanation for the Yeti.

Bear species are known to walk on two hind legs,

so we could still be seeing where these prints come from.

[intense music continues]

- [Laurence] The Himalayan mountain range stretches

for more than 1,500 miles

across five countries.

Any one of these countries

could hold the answer

that finally solves

the mystery of the Yeti.

But some researchers believe

that the most compelling clues come from China.

- As far back as around 300 BCE

one of the original accounts is a Chinese dictionary

that mentions one of these wild men

that have like the face of a human

and the body of a beast.

- In the 16th century, during the Ming Dynasty,

a naturalist named Li Shizen describes different types

of wild men that exist all around China.

- Now, you have to understand that China

is an incredibly large country,

and all the different mountainous regions

all start to yield their own kind of legend

about the wild men.

[intense music]

Over the years, these Chinese legends begin to coalesce

and form one story, one mythology,

one legend, one bit of folklore.

And this legendary creature now becomes known as the Yeren.

- One of the most well-known stories is that these wild men,

if you come across one,

they will be so filled with joy

at coming upon you that they will bear hug you

and pass out from excitement,

and you'll fall to the ground with 'em

'cause they're so strong, you can't get away.

And then when it wakes up, it eats you.

[intense music continues]

- However, there are some researchers

who speculate that the Yeren

might not entirely be based in myth and in folklore,

and that, in fact, it might be based

off of a species that existed on the planet,

in reality, a long, long time ago.

[intense music continues]

- The name Yeren comes from the Shennongjia region of China,

where in 1555, a local newspaper reported

that wild men were living in the forest

just outside of town,

and they were coming into town

and stealing people's dogs and chickens to eat them.

- It would be really easy to dismiss these stories

as being a relic of the past,

an old wives' tale,

but sightings persist.

- In 1976, six officials

from the Chinese Regional Forest Committee

are walking along in the woods,

and they come across these wild men.

And they get within a few yards of them,

and they describe them

as being tall, fully covered in hair,

and having facial features

that are a mix of humans and ape.

[suspenseful music]

- In 2007, some tourists

that are in this Shennongjia region

say they saw these two large figures

that are covered in hair,

and as they tried to approach,

those individuals ran into the forest.

They go back, they get forest rangers,

and together they go back to figure out exactly

where these creatures went,

and they find broken branches, some footprints,

and even half-eaten fruit.

[birds chirping]

- It's hard to say

that 2,000 years of eyewitness accounts

is all just mistaken identity.

So really what is happening here?

[suspenseful music continues]

[birds chirping]

[intense music]

- [Laurence] One theory that still intrigues researchers

to this day first emerges back in 1935,

thanks to a Dutch anthropologist

with an interest in the Far East.

- Ralph Von Koenigswald is in Hong Kong on a research trip,

and during the day he stumbles into a medicine shop.

And while he is in there,

he sees, in a jar, a giant molar

labeled "Dragon's Tooth."

- The reason they're selling this tooth

is because it's purported

that if you grind up this tooth

and you ingest it,

it's supposed to give you magical powers.

But Von Koenigswald looks at that tooth

and he says, "I know this tooth.

I know where this comes from.

This is a primate tooth."

So he buys the tooth.

He takes it back.

Does research on it.

- And through his research,

he identifies it as an early hominid species

from the human family tree.

According to him,

this particular species would've been far back enough

in the human family tree

that it would've been completely covered in hair

except for its face and its hands,

kind of like a chimpanzee,

except this chimpanzee would've been about 10 feet tall

and weighed upwards of 1,200 pounds.

- [Laurence] Based on the primate's immense size,

the species is named Gigantopithecus.

- Fossil discoveries over the last eight or nine decades

show that Gigantopithecus lived in the area

that is China and Tibet over 2 million years ago.

Most scientists agree

that Gigantopithecus went extinct about 250,000 years ago,

but there are those of us that believe,

because of the remoteness of the region

and their ability to adapt,

they have actually thrived in the high-altitude environments

of China, Tibet, Bhutan, and Siberia.

- In the 1950s, a British zoologist,

he looks at Shipton's photographs,

and he creates a plaster mold of what he thinks

that footprint would've looked like.

- After he examines this plaster cast,

he determines this could not have been made by a bear.

It had to be made by a very large bipedal hominid,

and he determines that it is likely the Gigantopithecus.

- [Laurence] Other researchers soon voice their support

for the Gigantopithecus theory.

- 1977, American anthropologist Grover Krantz believes

that the Gigantopithecus could have survived

in a small tribe in the remote areas of the Himalayas,

and, therefore, could be the basis

for what the modern Yeti is.

- [Laurence] It's possible there are even more layers

to the origin story of this ancient primate.

- Researchers and scientists

have found very, very few fossils

when it comes to Gigantopithecuss,

and because of that, it's very hard to suggest

that there's a tribe of Gigantopithecus

that hangs out in the mountains

and evolved into a new species

that became the basis for the Yeti.

- Until we have something like a complete skeleton,

it will remain in the realm of speculation.

Further research into the Yeti legend suggests

that perhaps Yeti is an even closer relative

to humans than we had previously thought.

[intense music]

- For centuries researchers attempting

to solve the mystery of the Yeti

have relied on inconclusive

or circumstantial evidence to try to build their case.

Despite some compelling pervasive oral history,

the hard proof has not come easily.

But in 2011, a Russian scientist

announced an intriguing discovery in Siberia

that could completely change the narrative.

- Dr. Igor Burtsev claims to have found footprints in a cave

in the Kemerovo region of Siberia,

which is just north of Mongolia,

footprints that he says belong to the Yeti.

[suspenseful music]

- In the cave, researchers not only find the footprints,

but they find what could be bedding

laid out as if something was staying there.

And in the bedding they find these long gray hairs.

DNA tests on the hairs confirm

that it is not from Homo sapien,

but it is from another hominid species,

one that is closely related to Homo sapien.

[suspenseful music continues]

- Burtsev concludes that the gray hairs

that were found in that cave

actually belong to Neanderthals

that are still living in these mountain ranges in Russia.

[suspenseful music continues]

- Burtsev is not the only person

to tie the Yeti to Neanderthal.

This is something that's been around for a little while

in the Russian academic world.

- [Laurence] Burtsev believes a number

of recent Yeti sightings in Siberia support this claim.

- In September 2012, there's actually three sightings

in the Kemerovo Mountains of Siberia.

In fact, one of the biggest was a fisherman.

He's cruising along and he sees

what he thinks are just

a couple bears at the shoreline drinking water,

but as he gets closer,

they actually stand up

and run off into the forest.

[suspenseful music continues]

- To date, the most compelling evidence we have

comes from a short video

that was taken in February of 2013.

Some boys are playing along

the Ur River in the Kemerovo region,

and they see this large bipedal hominid

walking through the trees

carrying what looks like

to be offspring or young.

And as they're excitedly filming this, very shakily,

the creature turns and looks at them,

and they lose it.

They lose it, and they run.

- [Laurence] But were these various encounters

actually with Neanderthals or something else?

- Neanderthals are a very distinct species

that showed up around 400,000 years ago

in the area of the world that's now known as Eurasia.

There was a period of time for about 30 to 50,000 years

where Neanderthals and Homo sapiens did coexist.

- The difference between Neanderthal

and humans are that we are very thin, our builds,

where they were very robust

and built for cold climates.

They had thick bones, a big rib cage.

They have elongated skulls,

large, wide noses, and like a thick brow.

[suspenseful music continues]

The scientific community believes

that Neanderthals went extinct about 40,000 years ago

due to the fact that they were competing

with humans for resources.

- However, there are some researchers that do suggest

that Neanderthals didn't go extinct

and that they continue to live on

in very isolated regions of the planet.

- It makes sense if you're a Yeti living in northern Russia,

that you could basically stay out of view of humans

because it's extremely remote, extremely cold,

and there's very few humans.

It's a place where these Yetis or Neanderthals

would actually be able to exist peacefully.

[fire crackling]

- If the majority of Neanderthals

went extinct 40,000 years ago,

save for maybe this small group,

that means that there have been 1,000 generations

of Neanderthals interbreeding with each other,

and as they interbreed with each other,

those more brutish physical features

become that much more pronounced.

[tense music]

You start to develop a kind of hominid

that is even hairier, maybe even stockier,

maybe even bigger, more barrel-chested.

- The same qualities that could contribute

to why a Neanderthal species would be able

to survive in these climates

could also explain the sightings of a Yeti creature today.

[tense music continues]

- The fact that the Yeti seems

to vanish almost immediately once humans come upon them.

Neanderthal would be very adept at hiding,

especially if they know in their collective history

that humans are the reason

there's only a small handful left.

Like the Himalayas, Siberia is incredibly remote.

The possibility of a intelligent species

staying hidden makes sense.

- You would need definitive evidence

before you can say that this theory actually holds water.

But it's an interesting idea.

[tense music continues]

- [Laurence] Researchers attempting to identify the Yeti

often focus on an unknown animal or a lost species,

but some investigators contend

that what people think they're seeing

is actually a kind of mirage.

- We can't say for certain

that there are a lot of Yetis out there,

but there are absolutely a lot of sightings.

[intense music]

So what are people seeing?

We ask that question

and try to answer it through the archaeological record,

through the fossil record,

but we could also answer that question with psychology.

[intense music continues]

- In 1998, famed mountaineer Reinhold Messner

decided that he was going to write a book about the Yeti

and get down to what is the Yeti really?

- Reinhold Messner was the first person

to do a solo ascent of Mount Everest.

He was also one of the first people

to do it without any supplemental oxygen.

He is someone who knows

what he's doing in mountain climbing.

- He interviewed as many people as he could,

sherpas, Bhutanese, Nepalese, everyone.

And what he came up with

was he believes people are not actually seeing

a flesh-and-bone animal.

What they're actually experiencing

is something called pareidolia.

[suspenseful music]

- A phenomenon called pareidolia,

which essentially is the human brain finding patterns

and seeing structures and connections among designs

that aren't actually there.

For example, why individuals have seen the Virgin Mary

in a piece of toast,

or we see figures in the clouds.

- When you go way back in deep time

when, you know, we had things to worry about,

like a saber tooth cat, every second counts.

And so that puts our mind into like a defensive trick.

And so the pareidolia is basically

your mind trying to give you an extra split second

to get outta the way of danger.

But a lot of times it's just a false alarm.

- [Laurence] But that isn't

the only psychological phenomenon

that could explain the Yeti.

[suspenseful music continues]

- Another possible optical illusion

that they could be experiencing

when having a Yeti encounter are shadow people.

[suspenseful music continues]

And shadow people are your mind playing tricks on you.

- A shadow person is literally you perceiving

that a shadow is a threatening humanoid

or a threatening entity.

Example of this is you wake up in the middle of the night,

and you look at the foot of your bed,

and you see this dark figure.

[suspenseful music continues]

You shake it off a little bit,

and then that shadow person is now vanished.

[suspenseful music continues]

- What's interesting about that

is it depends on your belief systems.

People who believe in the paranormal or supernatural

are far more susceptible to seeing

or experiencing shadow people.

- However, not all shadow people

are figments of the imagination.

There is another phenomenon

that is known as Brocken spectre,

and you look in the distance,

and you do see a menacing shadow, a menacing figure.

But Brocken spectre occurs when you're standing,

and the sun is behind you,

and it's essentially projecting your shadow in front of you.

- If the wind is blowing,

if that mist is moving at all,

the shadow might appear to move in ways totally unrelated

to the person whose body is casting that shadow.

In really high elevations,

especially where the air is really thin,

the Brocken spectre becomes even more common.

There's more opportunity for the right angle of the sun

and more opportunity for a substance

or a surface on which the shadow can be cast.

It's not hard to imagine

that people seeing this apparition

might attribute some sort

of supernatural significance to it.

- Eric Shipton himself,

the British mountaineer who found the 1951 prints,

he experienced a Brocken spectre

while climbing Mount Kenya in 1929,

and he describes it as seeing the shadowy figure

with this beautiful rainbow light cast around it.

What's easier to believe,

that people are seeing a giant relic hominid

wandering the mountains

or their minds are simply playing tricks on them?

- Scientists speculate there is

yet another possible explanation

for the sightings of Yetis at these altitudes,

and it is less a question of a psychological stress

and more a question of physical stress.

[intense music]

- [Laurence] Over the centuries,

the vast majority of Yeti sightings

have occurred high in the mountains,

and some researchers suspect there's a reason why.

- Humans have evolved

to live pretty much right at sea level.

And so when we go higher

and higher up in elevation,

we lose that concentration of oxygen.

- When you have people that are climbing mountains

like Mount Everest or Mount Kilimanjaro,

they usually will make base camp at around

anywhere from 17 to 19,000 feet,

and they'll stay there for a few weeks

just to acclimate themselves to that higher altitude

so that as they go higher, they don't get sick.

- But if we ascend to high altitudes too quickly,

that is when our bodies will begin to fail us.

You can see taste, touch,

and fully experience something

that is completely a figment of your imagination.

[intense music]

- Altitude sickness is no joke.

It can actually lead to death.

Symptoms of it are things like headaches, nausea,

dizziness, disorientation,

and also hallucinations.

- [Laurence] Hallucinations, which could explain

the long history of Yeti encounters in the Himalayas.

- Hallucinations are sensory experiences

entirely generated from within the brain.

So it's important to note

that hallucinations aren't just imaginary.

It feels, looks, sounds, smells real.

[intense music continues]

When someone experiences a hallucination,

it's like a psychosis.

There's no distinguishing between reality and fantasy.

- Someone suffering from altitude sickness

may not even be aware that it's happening to him

because the symptoms can come on very slowly and subtly.

One second, you're there and you're feeling great

and the next second you are talking to an imaginary person.

- There is an example of this

that takes place in 2008.

It's a mountaineer by the name of Jeremy Windsor.

Jeremy Windsor is attempting to climb Mount Everest,

and as he gets higher and higher,

he suddenly has this companion that's climbing with him

by the name of Jimmy.

- And Jimmy sort of keeps pace with him.

He sees him kind of out of the corner of his eye,

over his right shoulder.

They encourage each other

as they're struggling to make it to the top.

- When he gets back to base camp

and starts to explain his friend Jimmy,

it becomes very apparent, not only to his companions,

but to him, that Jimmy was a complete figment

of his imagination.

- Hallucinations often vanish as quickly as they appear,

which could explain why people are seeing the Yeti,

because the Yeti seems

to vanish right when you get a good look at it.

- [Laurence] Researchers also believe

that our hallucinations can be influenced

by deep-seated cultural and personal beliefs.

- Hallucinations happen.

We know that.

It's a human biological, physiological experience,

but how we interpret those hallucinations

owes a lot to culture.

Folklorists actually call this

the cultural source hypothesis

when it comes to explaining a supernatural belief

that someone has.

[expansive music]

- If you're in the Himalayas,

and you've been hearing about stories of the Yeti,

your subconscious might actually drum up a Yeti

for you to hallucinate

as you're making your way up this mountain.

- We know that at about 8,000 feet,

the vast majority of people do not feel any symptoms

of altitude sickness.

When you get to 10,000 feet, however,

about 75% of the human population

does start to experience evidence

of this altitude sickness, including hallucinations.

- So with the majority of Yeti sightings

happening at above 10,000 feet,

it's reasonable to say

that these sightings might just be

a symptom of altitude sickness.

- [Laurence] But not everyone is convinced

this mythic creature is purely made up.

- It's hard to actually refute the fact

that we have the same,

if not just similar experiences over thousands of years.

So it makes it very difficult to just discount it all

as some sort of hallucination

versus something that actually might exist.

The fact that we don't have any concrete clues

over thousands of years

lets the skeptics basically weigh in

and say, "Doesn't exist."

But biologists discovered the Coelacanth,

thinking it was extinct

tens of thousands of years ago.

- It is also perfectly plausible that no single explanation

for the Yeti is the entire explanation.

Human beings and science are also continuing to evolve.

So it's very plausible that the conclusive piece of evidence

that is needed to definitively prove the existence

of the Yeti is just around the corner.

[intense music]

- There's something uniquely compelling

about a wild, hairy humanoid

prowling in the cold shadows

in the highest peaks of the Far East.

It's a campfire tale that still inspires wonder

and dread to this day.

Is it a real life relic,

a trick of the mind, or something else?

The physical evidence may not be there,

but the intrigue lives on.

I'm Laurence Fishburne.

Thank you for watching "History's Greatest Mysteries."

[intense music fades out]