Original subtitles

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 Yucatán.

So, all of the movement of water

in the Yucatán, practically all the movement

of water in the Yucatán 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 Yucatán 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 Yucatán 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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