All language subtitles for The UnXplained s07e18 Mysteries of the Underground.eng

WILLIAM SHATNER:
Massive caverns

that stretch for miles
into the darkness.

Underwater caves believed to be
portals to the underworld.

And an ancient
subterranean wonder

that may hide the remains
of a monster.

Just below Earth's
vast and diverse landscape

lies thousands of tunnels
and caves.

While some have been carved
by the forces of nature

over millions of years,

others were painstakingly
created by human hands.

Since the dawn of mankind,
underground passageways

have been used as a place
to perform secret rituals

preserve human remains
and, according to some,

serve as gateways to the realm
of the supernatural.

What mysteries of
the underworld are hiding

in the dark recesses below us?

Well, that is what
we'll try and find out.

? ?

SHATNER: It's sometimes hard
to believe that on a planet

populated by
eight billion people,

there could still be places in
this world yet to be explored.

But such is the case with the
majority of caverns and caves

that ramble and weave
deep inside Earth's surface.

JILL HEINERTH:
Very few caves on the planet have been explored

because they're difficult
to get to.

There'll be caves deeper than
any human has ever been before,

and in remote places and
mountaintops we can't yet reach.

It's, um, also taboo in some
cultures even to go into caves.

I've been cave diving
all around the world

and when I get
to explore a cave,

I'm often the first and only
person to ever go inside there.

Probably 90% of the caves
on this planet

haven't even been explored yet

and yet, we're finding
so many unique things.

Caves have so much to teach us,

both dry caves
and underwater caves.

Even the species that
we recognize inside caves

behave differently.

I mean, we're talking about
a completely dark,

food-scarce environment,

and yet, there are animals that
are living within these caves

that have not evolved
in more than 65 million years.

We've already sequenced

more than 4,000 different types
of bacteria growing in caves.

And these are sources for future
chemicals, for future medicines.

We also have astrobiologists

that are interested in
the microbes that live

in these really harsh
environments of caves

and, um, this could be the clue
to life on other planets.

SHATNER:
While caves offer boundless potential

to unlock the natural secrets
of our planet-

and even worlds beyond our own-

they also hide the story
of mankind's ancient past.

For instance,
Wonderwerk Cave in South Africa

has archeological evidence

that our ancestors lived
in caves for two million years.

But just as it was
for ancient man,

Earth's mysterious underground

is still one of
the most dangerous

environments imaginable.

There are a lot of hazards
involved in exploring caves

because, uh, uh, you're going
into an environment

that's completely dark...

...and you have to have
the right equipment.

If your batteries run out,
then you're screwed.

If you're far back in a cave

and you don't have a light
to get out.

A lot of caves involve a
significant vertical component

to the exploration

where you have to rappel down
into them,

and you have to use ascenders
to get out, rope ascenders.

I was on this one expedition
into a cave in New Mexico

where, uh, one of the people
on my... on-on my team

got stuck and couldn't get out.

And so, we had to send a team
to the surface

to get tools to bring
back down in there

to chop away at the rock
to get him out.

Because we knew it would
be several hours

before a rescue team
could get there.

If it's a submerged cave,
the danger is even greater.

It's not uncommon
for the floor of the cave

to be covered with silt.

And if you get too close to it
and your flipper hits it,

it'll silt up,
you'll get a complete whiteout.

You won't be able to see,

your visibility
suddenly goes to zero.

I've heard stories
about cave divers who drowned

and they were only about 50 feet
from the entrance to the cave

because they-th-th-they had
no visibility at all.

SHATNER:
Exploring Earth's underground

sounds like a potentially
deadly proposition.

But despite the inherent risks,
every so often

a new cave system is discovered

that takes your breath away from
its sheer beauty and wonder.

SHATNER:
Local lumberjack, Ho Khanh,

searches the dense jungle
for agarwood-

a valuable fragrant wood
found in Aquilaria trees-

when he stumbles upon
something monumental.

(translated): I went into the
forest to search for agarwood.

One afternoon,
when it started to rain,

I sought shelter.

I found a mysterious
cave entrance,

which was unlike any
other caves in the area.

The entrance was covered with
a thick layer of mist and steam,

and there was
wind blowing out from it.

I'm the very person
who discovered Son Doong Cave,

the world's largest cave.

SHATNER: Ho Khanh's remarkable
discovery was so difficult to find,

it took him 18 years
to properly retrace his steps

and locate the caves
hidden entrance once again.

And in 2009,

he successfully shared
the remote underground world

with British cave explorer
Howard Limbert.

To get to Son Doong Cave
it's-it's quite an expedition.

It's not next to the road.

It takes a day and a half walk.

And the only way
to get to Son Doong

is to come through Swallow Cave.

And it's where we sleep
the first night.

And the next day
we go through this cave,

through a hidden valley
to get to Son Doong.

That's the only way
into Son Doong.

So, it's quite an adventure
to get just to the entrance.

But the number of people
that have been in Son Doong

is less than
the number of people

that have climbed Mount Everest.

We're very strict on
conservation within the cave.

We restrict the number of people
that come into Son Doong

to protect the cave.

Thus not many people have
a chance to visit Son Doong.

It certainly is one of
the Great Wonders of the World,

and it should retain its beauty
for all the generations ahead.

BEN McGEE:
So visualize this

journey to the center
of the Earth kind of place

with its own forest,
and its own sunlight,

its own ecosystem,
its own river.

And it is so big,

you can fit
76 Mount Rushmore's inside

or 15 Pyramids of Giza.

This really is more
than a city underground.

LIMBERT: We found seven new species
in the last few years in Son Doong.

Such as white fish,
a cave spider,

and other small creatures
we found in the cave.

Also, it's got some
incredibly rare plants.

There's one plant that's only
seen as a pressing in a museum.

So, there's lots and lots
of rare things

within Son Doong
that we've found.

And, in fact,
there's still new passages

to be found in Son Doong.

We've dived the very end
of Son Doong

and it's still continuing.

But we need special gases
so we can dive deeper.

But there's a good chance
we can join Son Doong

with another cave,
which will make it even bigger.

The beauty about
exploring caves like Son Doong

is you don't know
what you're going to find.

So, we don't know what the next
discovery's going to be.

SHATNER:
Beneath the rolling hills,

deep river valleys
and dense forest

lies one of
the most awe-inspiring

underground environments
on planet Earth.

It's a UNESCO
World Heritage Site

visited by
600,000 people every year

called Mammoth Cave
National Park.

SCHROER: Mammoth Cave is the
longest cave system in the world

at around 426 miles.

Now, that is
what we know of today.

We are still mapping
Mammoth Cave.

There's still more cave to be
found, more passages to explore.

So every year, we just
add on to that mileage,

and at this point,

there's no end in sight.

It leaves a lot
to your imagination

to look out into
the darkness and wonder

what could be out there.

I've been in Mammoth Cave
easily more than 500 times.

It's a very complex cave system.

And it's got, very,
very old levels that are dry,

and then the most recent
levels have active rivers

flowing in them.

Mammoth Cave
looks like a big maze.

And there
are numerous cave systems

that could potentially
connect and make it longer.

So, we think that Mammoth Cave

has the potential
to be a thousand miles long.

And so, I'm anticipating
that I will find more cave.

SHATNER:
While scientists continue to map and explore

the uncharted depths
of Mammoth Cave,

there is a different
kind of darkness

that dwells in Kentucky's
famed underground...

...a dread that hovers
over this cavern's history

of tragic deaths.

The most well-known
tragedy involved

a legendary cave explorer
from the early 1900s

named Floyd Collins.

SCHROER:
Floyd Collins was not afraid

to go through
the really tight passages,

or to go where nobody
had ever been before.

One of the big discoveries
Floyd had in his time

was a cave called Crystal Cave.

And it is one of the most
beautiful caves that we have

in Mammoth Cave
National Park right now.

In 1925, he entered into
a cave known as Sand Cave.

Unfortunately, Floyd went in
in late January,

and he did not come back out.

No one knew where Floyd was
until they went exploring

outside of Sand Cave
and they saw Floyd's jacket.

His brother yelled down,
"Floyd, are you down there?"

And Floyd yelled out,
"Help me. Get me out of here."

It was a big national
news story of the day.

He was just stuck.

He was unable to move his arms.
He couldn't reach his feet.

They described it as
a limestone straightjacket.

It's just a terrifying thing

to think of being stuck
inside the cave.

And they gave as much effort
as they could to get him out.

After two weeks of
a statewide, national

and local effort
to try to get Floyd out,

he unfortunately died
inside the cave.

KAMBESIS: Mammoth Cave does
have a reputation of being haunted.

There's lots of stories
about people

having experiences in the cave
that they can't really explain.

I mean, I've had
an experience where

I thought I felt,
like, a presence there.

That's a little bit unsettling.
(laughing)

Lots of people,
they're convinced

that it's Floyd Collins' ghost
that's haunting that cave.

I mean, his coffin was
in Crystal Cave for decades

before they had a proper burial
and put him on the surface.

SHATNER: Is Mammoth Cave really
haunted by the spirit of Floyd Collins?

While many certainly
believe it's true,

there are other tragic
deaths that occurred

in Kentucky's subterranean world

that may also be the source of
lingering, restless spirits.

There was a doctor
named John Croghan

who purchased
Mammoth Cave in 1839

and there was this thought
that maybe the different air

in the cave could
actually cure tuberculosis.

And so, he had
an entire facility

built inside Mammoth Cave,

where they would
take critically-ill

tuberculosis patients,
with the hopes

that they would get better.

And of course, they didn't.

After a few years,
everyone died.

SHATNER:
While their deaths certainly add another layer

to Mammoth Cave's
haunted reputation,

it's possible that spirits
have inhabited this cave

for thousands of years.

Because in the early 1800s,

miners were surprised
to discover that the cave

was once an ancient
burial ground.

People began finding evidence
not just of prior mining

but of prior ritual use.

One of the finds was
what is clearly a crypt.

Inside was the mummy
of a Native American woman.

She was preserved
almost perfectly.

According to accounts,
she was draped in deer skins

that were decorated with leaves
and vines and things like this.

This is really clearly
a thoughtful ritual burial.

She did get the name Fawn Hoof,
because she had a necklace

of-of hooves from fawns.

And she apparently was
buried with a lot of items

and plants that you might
expect of someone

who was a healer
to carry with them.

She would have been a person
of high status in the society,

someone looked up to.

She was buried with things
that they may have thought

she needed for an afterlife.

SHATNER: It was reported that
several other Native American mummies

were dug out of Mammoth Cave,

including one found in 1935
nicknamed "Lost John,"

who would eventually
be displayed inside the cave

as a tourist attraction.

Many have wondered,
could the desecration

of this ancient burial ground
and its mummified human remains

explain the otherworldly
presence reported here?

The scientists estimate
that some of the remains

that were found in the cave
date back 5,000 years.

Babies have been found in there,
one which was brought out

and, within a few hours,
completely disintegrated.

You have to understand
that the caves

would have been
considered sacred

by these different
tribes in the area.

In many
Native American cultures,

caves are the entrance
to the underworld.

These are places where you go

to commune with
the spirits of the dead.

There's catastrophic loss
of human life in there,

and a lot of mysticism
that goes along with it.

And at the time,
these mummies became attractions

for tourists to come look.

KAMBESIS:
Every once in a while,

you get a sense
that there's a presence there.

You'll start to hear things,

like there's something there
besides you.

And sometimes, it's scary.

But many cave explorers
that I know

have explained the same thing.

That there's an unexplained
phenomenon in this cave.

So, what do you think?

Could the discovery
of mummies at Mammoth Cave

have riled the spirits
of those laid to rest there?

Perhaps some places
are best left undisturbed.

Like in the case of
an underwater cave system

in Mexico that may be guarded

by a powerful serpent.

SHATNER:
A six-mile-wide space rock

smashes into Earth...

...and sends billions of
tons of rock and debris

into the atmosphere,
generating tsunamis

that are half a mile high.

This apocalyptic impact

kills 75% of all life
on the planet,

and creates
about 6,000 sinkholes

and underwater cave systems
known as cenotes.

Cenotes are one of the principal
sources of water in the Yucat�n.

So, all of the movement of water

in the Yucat�n,
practically all the movement

of water in the Yucat�n
occurs in the subsurface.

A cenote is literally
a hole in the ground,

and it's usually
flooded at the bottom.

The ancient Mayans
used the cenotes

on the Yucat�n Peninsula
for a water supply.

And the cenotes also
have cultural significance

because of the role that
they played in Mayan religion

and Mayan agriculture.

SHATNER: Cenotes were sacred
to the ancient Mayan civilization

who existed in the region
for about 3,500 years

starting around 2000 BC.

These natural wells
of freshwater

were essential for growing crops
and clean drinking water.

In fact, one of the Mayan's
most important cities

on the Yucat�n Peninsula
was called Mayapan,

and it was
purposefully constructed

around these vital
water sources.

LYNNE McNEILL:
Mayapan was a really

sprawling, beautiful city

built entirely
inside a protective wall.

And within that wall
that surrounds it,

there are a number of cenotes.

Which, of course, if you're
an ancient civilization

looking for
a fresh water supply,

this would have
been a phenomenal place

to build your walled city
and make sure that you contain

all of those bodies of water.

But what's really interesting
is that there is one cenote

that is outside the city walls.

And this particular cenote
is known as Sac Uayum.

And it's not incidentally
outside the city walls.

The city walls actually zigzag

in this really awkward way

around this one pool of water.

SHATNER:
Why would the ancient Mayans

go through all the effort
to build around

this one cenote
known as Sac Uayum?

Perhaps answers can be
found with strange tales

that modern-day descendants of
the Maya still tell to this day.

RUSSELL: We've investigated
dozens of cenotes across the site.

There's several dozen within
the city walls of Mayapan.

But we began to hear stories
about, you know, the cenote

that was outside of
the city wall

that seemed to be
kind of deliberately excluded.

There's several kind of stories
I was hearing about Sac Uayum.

It's guarded by this...
this important

and-and powerful being,

that it was protected
by a feathered serpent.

That people had seen
the feathered serpent

up in the trees and watched
it kind of fly up in the air

and dive down into
the cenote when they approached.

SHATNER: In 2013, anthropologist
Bradley Russell and his team

prepared to learn more
about this storied cenote

and its hidden secrets,
and descended into Sac Uayum.

RUSSELL:
It's about four stories down to water.

So we knew we were
gonna have to rappel in,

get in the water, dive.

And we noticed that
there was a small opening,

there was a tunnel
that would lead us to something.

So, you know, you went
from this large open chamber

into this narrow tube,
and then you pass through that,

and it opens up

into this massive
beautiful white-walled

gorgeous second chamber

that's filled with water
the whole way to the ceiling.

We began to explore that,
and we realized

pretty quickly that there were

human remains
in the second chamber,

including several skulls
that are sitting just completely

face up, staring right at you.

It was unusual because
there was no logical way

that you could
have put a body in there.

We didn't really
understand at first

how there could be
human remains in there.

It turns out
that the first chamber

sits a little bit above
a section of the lower chamber.

And what seems to have
happened was that the floor

of the first chamber caved in,

brought down stone
in kind of a slope,

and then human remains

that are lighter than
the rock settled out.

When we analyzed samples
of the human remains

back in the lab, we do a count

that we call the "Minimum Number
of Individuals," the MNI.

And the MNI we came
away with was, like, 19.

So we know that there's close to

a couple of dozen individuals
that are there.

And when we did
radiocarbon dating

on their bones that we had
across the collection,

they fell consistently
about 1350 AD.

We found that it was
mixed age and mixed sex.

So, every member of society,
essentially,

was represented
relatively equally.

HEINERTH:
The Mayans considered cenotes as the entryway

to a place they called Xibalba,

which was really their form
of the underworld or hell.

So, cenotes
were sacred places where,

sure, there were
accidental drownings,

but there were also
intentional sacrifice rituals,

potentially to seek their gods

to send water
in a time of drought.

SHATNER: It's disturbing to
consider what may have happened

at Sac Uayum to
appease the ancient gods.

But for now,
the mystery surrounding

this underwater
underground world endures.

RUSSELL:
We didn't find any evidence

for decapitation
or things that might be

the violence of
the actual sacrificial act.

So, we were needing
another explanation.

And what we've concluded is that

there was probably
a series of disease outbreaks

around 1350 AD,

and somebody needed to deal with
the bodies quickly en masse.

And people chose
cenote Sac Uayum

to put the bodies into.

So the mystery
of why they're there is

slowly being clarified,

but I would never
say at this point

that we're 100% certain
that that's what it is.

Could a pathway to
the underworld really exist

in the watery caves of Mexico?

Given their history, it seems
like a chilling possibility.

But there's another cave,
this one man-made,

that is said to have been
intentionally created

as a place to worship the Devil.

SHATNER: One of the most powerful
political figures in Britain,

Sir Francis Dashwood,

commissions the excavation
of a quarter mile

of tunnels and chambers

that extend 300 feet
below his property.

These man-made caves would soon
serve as the meeting place

for Dashwood's
enigmatic secret society

known as the Hellfire Club.

Their underground headquarters

would be aptly named
the Hellfire Caves.

The Hellfire Caves are
quite a mysterious place.

You know, they're subterranean.

They go under West Wycombe Hill
about 300 feet.

When one enters
into Hellfire Caves,

the first thing you see

is the great gothic facade
on the entrance,

and that kind of really
sets the tone of this darkness

and mysterious nature.

And as you come further down
into the cave network,

there's very narrow passageways,
very narrow doorways.

And then all of a sudden,
you find yourself

in areas such as
this central cavern,

which are very ornate and very
kind of precise and deliberate.

You do get that sense
of mystery and myth

and legend going on here.

So, here we are
in the banqueting hall

of the Hellfire Caves,
and as the name suggests,

this is where
the Hellfire Club members

would've wined and dined

before their other activities
later in the evening,

such as ceremonies and rituals.

The Hellfire Clubs were these
extraordinary secret societies

in the 18th century
that were set up

by, basically,
the cream of society.

At these
secret society meetings,

they're questioning
the monarchy,

they're questioning the church,

they're questioning
old, established ideas.

They were worshiping pagan gods.

And so, Dashwood creates
a really extraordinary

venue for these meetings.

SIR EDWARD DASHWOOD:
It was secretive.

It was certainly exclusive.

And the sort of membership
that went there

and were members
were people like

Lord Sandwich, who was
First Lord of the Admiralty.

Today, he'd be the equivalent
of the Minister of Defense.

You had Thomas Potter,

who was the Archbishop
of Canterbury's brother.

Benjamin Franklin visited.

I wouldn't know
if he was a member,

but he certainly
visited the club.

And then you had my ancestor,
as well, Francis of Wycombe.

And Sir Francis Dashwood
was definitely an intellectual,

and he always felt
there was some

better meaning behind religion.

SHATNER: What was Sir Dashwood
and other members of high society

doing in the dark,
300 feet below the ground?

While it's been a matter of
debate for hundreds of years,

many believe the long,
secret parties

may have descended into
something much darker

and more disturbing.

So what went on at
these Hellfire Club meetings?

They essentially engaged in

a lot of blasphemy,
fornication and gambling.

They dressed up as monks
and bishops and essentially

mocked organized religions

through holding
satanic pagan rituals

behind closed doors.

-Were people sacrificed?
-(woman screaming)

Were people killed down there?

Those who spoke about
what they'd seen

talked about the Devil himself
chairing those events.

There was a lot of talk
of devils being present.

In amongst these claims
and counterclaims

about what happened,

there's a lot
that's unexplained.

So, here in the cave network,
there are loads of

inscriptions in the wall
depicting these kinds of faces.

Some of them are said
to represent ghosts, imps,

things like that,
these mythological creatures.

Some have suggested that these
are actually satanic in nature.

We'll never truly know what
the artist fully intended

when depicting these, but
there are some great examples

all throughout the network
of faces just like this.

SHATNER:
Was the Hellfire Club really engaged

in dangerous satanic rituals?

Or were they just
bored aristocrats

enjoying outrageous
drunken parties?

While it's impossible to say,

perhaps the truth lies within
the most secretive chamber

of the Hellfire Caves
called the Inner Temple.

DASHWOOD:
There are a series of tunnels

going 500 meters underground,

going downhill all the way
and you come to a river

and an inner temple
right at the bottom.

And the whole conceit
of this river

is that it's meant to be
the River Styx,

which, if you know
from your mythology,

separates the real world
from the underworld.

There's a bridge there now,
but in the old days

there was a proper separation,

and there was a boat
that took you across.

It might have only been
two or three meters,

but it was this whole narrative
that you are now entering

the inner temple,
or the underworld

as it was meant to be.

DARKWOOD:
The Inner Temple of the Hellfire Caves

is literally as far as you can
go within the cave network,

and as far as
the Hellfire Club is concerned,

it was the most secretive
chamber of the cave network.

Only the most elite members
of the club had access

to the Inner Temple.

DASHWOOD: There's a lot going
on that we probably will never know.

It was through, really,
the 19th century,

so a hundred years later,

that the stories of sort of
devil worshiping

and black magic
really came to fall.

And we can't prove
or disprove them.

How far it went, we don't know.

The Hellfire Club's
fascination with the devil

and the disturbing rituals

performed in
their subterranean lair

are further examples
that the underworld

is a perfect place
to hide secrets.

Which is the case with
an underground reservoir

in Turkey
that some believe once served

as the tomb of the mythical
monster known as Medusa.

SHATNER: For thousands of years,
this ancient city served

as the capital for the Roman,

Byzantine and Ottoman Empires.

And evidence of these
once mighty civilizations

can be witnessed
in the architectural wonders

that still exist today.

But deep below
the city's surface

lies one of the most mysterious

ancient structures
in the world...

...the Basilica Cistern built
in the 6th century AD

by the Byzantine Emperor
Justinian.

The Basilica Cistern
is the best preserved

underground cistern
probably in the world.

A cistern is a water storage
container of varying sizes

and the aqueduct that fed
the Basilica Cistern

spread throughout
the water systems of the city.

So, what we're seeing
above the city

in these grand constructions...

...is mirrored below the city
in these underground features.

And so, the water
would then be piped

into the Great Palace,

into fountains

and water was channeled
through the city.

The cistern
that we're talking about

was located at a very
low level of the city

and was unknown for many,
many centuries.

There's something very magical
about descending into a cistern

and seeing
the columns around you

and having that
firsthand connection

with places that we were
never meant to enter.

RYAN: The most unexplained aspect
of the cistern is its beauty.

There was no reason
to make this utilitarian space

that no one would ever see
look like a palace.

And they did.

It is supported by 336 columns.

They are marble and granite.

And the columns soar overhead
to these brick vaults.

The purpose of a cistern
is straightforward.

But why it's so elaborate,
so beautiful

is the greatest mystery
of the space.

SHATNER:
While the Basilica Cistern's grandeur

is certainly enigmatic...

...an even greater mystery lies

tucked away in its
northwestern corner

where there are
two curious columns

supported by massive stones,

skillfully carved to represent
the head of Medusa.

GERSTEL:
Medusa in ancient Greek mythology

was a significant figure.

She is the daughter
of a sea-god...

...and her hair is made of
snakes, that's what you see

when you look at the Medusa
heads in the cistern.

Of course, we know
that in ancient Greek mythology

she was beheaded by Perseus.

And she had the power
in the ancient world

to turn people into stone
who gazed at her face.

We often find heads of Medusas
associated with temples,

gates of cities or aqueducts.

She's placed in those locations
to ward off evil

and to protect those buildings.

For the Basilica Cistern,
the Medusa heads,

one might, therefore,
ask whether

their placement was intentional,

if there was some reason
that they were placed

in a certain part
of the cistern.

SHATNER:
Were these Medusa heads meant to somehow

protect the city's water supply?

While it's possible,

some believe that the strange
orientation of the heads,

with one upside down
and one on its side,

may provide directions
to a hidden tomb.

As evidence,
they point to a document

from the late 19th century,

the diary of Ottoman Sultan,
Abdul Hamid II,

where he discloses the cistern's
400-year-old secret.

McMAHON:
Many years ago,

the diary of Sultan
Abdul Hamid II was discovered.

And in the diary,
he makes a reference

to a visit by a delegation
from the Republic of Venice

400 years before,

to see the then Sultan
of the Ottoman Empire.

These Venetians
were able to tell him

where to find
the sarcophagus of the Medusa,

which was apparently
in the Basilica Cistern.

So, the reason that that head
of the Medusa was there,

the statue,
was because her body was there

in the Basilica Cistern.

Now the story goes that they did
find the sarcophagus.

They opened it up,
and the sight inside

was absolutely horrifying.

It was a decaying creature
with the head of a human

and a snake's body,
and everybody present

was pretty convinced
that that was the Medusa.

SHATNER:
According to myth,

Medusa was in fact mortal

so it does stand to reason
that she died and was buried.

But was the Basilica Cistern,

this magnificent
subterranean reservoir,

really an elaborate tomb
for this legendary monster?

It's a fascinating claim
that at least for now

remains a mystery
carved in stone.

GERSTEL:
Obviously, the role of the Medusa heads

still needs to be explained,
where they came from,

what they're doing there,
why they're placed together.

It seems so intentional

as if you are witnessing
some underground construction

that may have had
another purpose.

But I'm not surprised
that the mystery of the space

would give rise
to these kinds of stories.

It's just one of the most

spectacular places
on the planet.

SHATNER:
Since 2014,

mysterious sinkholes
that are hundreds of feet wide

have opened up in this remote,
frozen landscape.

Over 20 of these deep craters
have been discovered

in Siberia's Yamal
and Gydan Peninsulas.

And while some have proposed
that they are the result

of meteor strikes or even
extraterrestrial visitation,

the most recent theory
is truly explosive.

The sinkholes in the
Yamal Peninsula in Siberia

are formed in permafrost.

Um, and they were all sort
of in the neighborhood

of 50 meters deep,
about 160 feet deep.

Permafrost is
permanently frozen ground.

It occurs in Siberia
and Northern Canada.

And in Alaska,
in the Arctic regions.

And the permafrost
on the Yamal Peninsula

has been frozen solid
for about 40,000 years

at least and they can
abruptly explode.

So, the sinkholes
in the Yamal Peninsula

are formed by a completely
different mechanism

from most of the sinkholes
that we know about.

The sinkholes
are actually these craters

that result from an explosion
from below.

They're actually
exploding gas craters...

-(explosion)
-...and it's somewhat akin to

the popping of a champagne cork.

So, you have this gas
on the subsurface

that's causing these explosions.

And that's what's creating
those craters.

McGEE:
Sinkholes are a phenomenon

that occurs all over the world.

There was the famous example
in Guatemala

in the-the center
of a street intersection

that was, uh,
nearly 300 feet deep.

That's a sudden sinkhole

that could have made
a 30-story building disappear.

Sinkholes can be extraordinarily
disruptive and dangerous.

So, some can be catastrophic

where there's a sudden failure
over a giant cavity

and then a massive hole
just makes things disappear.

And others are subtle,
slow burns

where there's many indicators
on the surface

as things get weaker
and subtly start to give way.

And then finally they'll
break and you have a giant hole.

There are few things
that we can encounter

that attach as much mystery
as something like a sinkhole.

Where there was something
before, it disappears.

It goes away.

SHATNER:
The truth is, we really don't know much

about the hidden dangers
lurking just below our feet.

And while there
are countless secrets hiding

in the Earth's
uncharted underground,

with every new discovery,

it seems the mystery
grows even deeper.

There's so much
about cave exploration,

so many questions
that we're still asking.

I mean,
we can learn about animals

that live in completely
different ways in the darkness.

We can learn about
how ancient cultures

have interacted
with these places.

About evolution, survival,
future chemical compounds,

pharmaceutical marvels,
and so many other things

that we haven't even
thought about yet.

So important, important,
um, discoveries

are coming from
cave environments

and the life within them.

McGEE:
The Earth has a way of surprising us.

It would be naive for
anyone to say, at this point,

that there are no mysteries
left to solve

when it comes to
any scientific venture,

not the least of which is,

"What's going on
right underneath our feet?"

Exploration under the earth

represents a frontier
in many directions.

There's a lot going on
down there we don't know about.

As we venture down
into the deep, ever farther,

we will start to find
the next layer of mysteries

left for us to discover
underground.

It is said that 90% of Earth's
caves and tunnels

are unexplored and with so many
new ones forming every year,

it would seem that
the subterranean world

is perhaps far too vast
to be fully understood.

Whether it's gigantic caverns,
hidden passageways

or massive holes that suddenly
appear out of nowhere,

the underworld is constantly
revealing new mysteries.

And no matter how deep we go,
one thing is clear,

we've really only
scratched the surface

of our planet's deepest,
darkest secrets

that continue
to remain unexplained.

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