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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!

The underworld are hiding in the dark recesses below us well That

is what we'll try and find out

It's

sometimes hard to believe that on a planet populated by 8 billion people

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.

Very few caves on the planet have been explored because they're difficult to

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

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

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 this could be the clue to life on other planets.

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, Vondervoort Cave in South Africa has archaeological 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

of the most dangerous environments imaginable.

There are a lot of hazards involved in exploring caves because you're going

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 the cave

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

A lot of caves involve a significant vertical component to the exploration

you have to rappel down into them and you have to use ascenders to get out,

ascenders.

On this one expedition into a cave in New Mexico, where one of the people on

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

there and 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 had no visibility at all.

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.

Quang Binh Province, Vietnam, December 1990.

Local lumberjack, Ho Khan.

searches the dense jungle for agarwood, a valuable fragrant wood found in

aquilaria trees, when he stumbles upon something monumental.

I went into the forest to search for agarwood. One afternoon, when it started

rain, I thought 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 Dong Cave, the world's largest cave.

Ocon's remarkable discovery was so difficult to find, it took him 18 years

properly retrace his steps and locate the cave's hidden entrance once again.

And in 2009, he successfully shared the remote underground world with British

cave explorer Howard Limbert.

To get to Sandong Cave, it's quite an expedition. It's not next to the road.

takes a day and a half walk.

And the only way to get to Sandong 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

Sandong. That's the only way into Sandong.

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

But the number of people that have been in Sondong 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 Sondong to protect the cave.

Thus, not many people have the chance to visit Sondong, which certainly is.

one of the great wonders of the world and it should retain its beauty for all

the generations ahead so visualize this journey to the center of the earth kind

of place with its own forest and its own sunlight on 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

We've found seven new species in the last few years in Sandong, such as

whitefish, a cave spider, and other small creatures we've found in the cave.

Also, it's got some incredibly rare plants. There's one plant that's only

as a pressing in a museum. So there's lots and lots of rare things within

Sandong that we've found.

And in fact, there's still new passages to be found in Sandong.

We've dived the very end of Sondong and it's still continuing.

But we need special gases so we can dive deeper.

But there's a good chance we can join Sondong with another cave which will

it even bigger.

The beauty about exploring caves like Sondong is you don't know what you're

going to find.

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

Edmondson County, Kentucky.

Beneath the rolling hills, deep river valleys, and dense forest lies one of

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.

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 caves to be found, more passages to explore.

So every year we just add on to that mileage. And at this point, there's no

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 1 ,000 miles long, and

I'm anticipating that I will find more caves.

While scientists continue to map and explore the uncharted depths of Mammoth

Cave, there's 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.

Floyd Collins was not afraid to go through the really tight passages or to

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

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

a limestone straitjacket.

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

out, he unfortunately died inside the cave.

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.

Lots of people, they're convinced that it's Floyd Collins' ghost that's

that cave. I mean, his coffin was in Crystal Cave for decades before they...

a proper burial and put him on the surface.

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.

They may also be the source of lingering restless spirits.

There was a doctor named John Krogan 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

critically ill tuberculosis patients with the hopes that they would get

And, of course, they didn't.

After a few years, everyone died.

While their deaths certainly add another layer to Mammoth Cave's history,

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

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.

is really clearly a thoughtful ritual burial.

She did get the name Fawn Hoof because she had a necklace of hooves from fawns.

And she apparently was buried with a lot of items and plants that you might

expect someone who was a healer to carry with them.

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

to. She was buried with things that they may have thought she needed for an

afterlife.

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

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

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.

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 beside you.

And sometimes it's scary, but many cave explorers that I know have explained the

same thing, that there's an unexplained phenomena in this cave.

So what do you think?

Could the discovery of mummies at Mammoth Cave have riled the spirits of

laid to rest there?

Perhaps some places are best left.

Like in the case of an underwater cave system in Mexico that may be

guarded by a powerful serpent.

The Yucatan Peninsula, Mexico, 66 million years ago.

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 a 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 Yucatan.

So all of the movement of water in the Yucatan, practically all the movement of

water in the Yucatan occurs in the subsurface.

The cenote is literally a hole in the ground, and it's usually flooded at the

bottom. The ancient Mayans used the cenote on the Yucatan Peninsula for a

supply, and the cenote also had cultural significance because of the role that

they played in Mayan religion.

And Mayan agriculture?

Cenotes were sacred to the ancient Mayan civilization, who existed in the region

for about 3 ,500 years, starting around 2 ,000 B .C.

These natural wells of fresh water were essential for growing crops and clean

drinking water.

In fact, one of the Mayans' most important cities on the Yucatan

called Mayapan.

And it was purposely constructed around these vital water sources.

Maya Pan 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 Weyum.

And it's not incidentally outside the city walls. The city walls actually

in this really awkward way.

around this one pool of water.

Why would the ancient Mayans go through all the effort to build around this one

cenote known as Sacwayum?

Perhaps answers can be found with strange tales that modern -day

the Maya still tell to this day.

We've investigated dozens of cenotes across the site. There's several dozen

within the city walls of Maya Pond.

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 Sacwayum.

It's guarded by this important 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

of fly up in the air and dive down into the cenote when they approached.

In 2013, anthropologist Bradley Russell and his team prepared to learn more

about this storied cenote and its hidden secrets and descended into

Saquayam.

It's about four stories down to water.

So we knew we were going to have to rappel in, get in the water, dive.

And we noticed that there was a small opening.

It 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

the ceiling we began to explore that we realized pretty quickly that there were

human remains in the second chamber including several skulls that are

just completely face up, staring right at you.

It was unusual because there was no logical way that you could have put a

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

And when we did radiocarbon dating on the bones that we had across that

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.

The Mayans considered cenotes as the entryway to a place they called Jibalba,

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

rituals, potentially to seek their gods to send water in a time of drought.

It's disturbing to consider what may have happened to Saquayum to appease the

ancient gods.

But for now, the mystery surrounding this underwater, underground world

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 A .D., and somebody needed to deal with the bodies

quickly, en masse.

And people chose Sonot de Saclayu 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

Could a pathway to the underworld really exist in the watery caves of Mexico?

Given their history, it seems like a chilling possibility.

There's another cave, this one.

man -made that is said to have been intentionally created as a place to

the devil.

West Wycombe, England, 1748.

One of the most powerful political figures in Britain, Sir Francis

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 Wickham 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...

being the areas such as this central cavern, which are very ornate and very

of precise and deliberate.

You do get that sense of mystery and myth and legend going on here.

But here we are in the banqueting hall of the Hellfire Cave. Another name

suggests this is where the Hellfire Club members would have 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

that were set up by basically the cream of society.

At these secret society meetings, they're questioning the monarchy,

questioning the church, they're questioning old established ideas.

They were worshipping pagan gods.

And so Dashwood creates... A really extraordinary venue for these meetings.

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, you'd be the

equivalent of the Minister of Defence.

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

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.

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 organised religion

through holding satanic, pagan rituals behind closed doors.

Where people sacrificed, where people killed down there, those who spoke about

what they'd seen talked about the devil himself chairing those events.

So there was a lot of talk of devils being present.

In amongst these claims and counterclaims about what happened,

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

this.

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.

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 metres, but it was this whole narrative

that you are now entering the inner temple or the underworld, as it was

be.

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

of the cave network. Only the most elite members of the club had access to the

inner temple.

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

It was through, really, the 19th century, so 100 years later.

But the stories of sort of devil worshipping and black magic really came

form. And we can't prove them or disprove them. How far it went, we don't

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.

Istanbul, Turkey.

For thousands of years, this ancient city served as the capital for the

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 A .D.

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,

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.

First -hand connection with places that we were never meant to enter.

The most unexplained aspect of the cistern is its beauty.

There was no reason to make this utilitarian space that no one would ever

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

The purpose of the cistern is straightforward, but why it's so

beautiful, is the greatest mystery of the space.

While the Basilica Cistern's grandeur is certainly enigmatic, an even greater

mystery lies tucked away in its northwestern corner, where there are two

columns, supported by massive stones, skillfully carved to represent...

The head of Medusa.

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

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 Medusa associated with temples, gates of cities,

or aqueducts. She's placed in those locations to ward off evil and to

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

in a certain part of the cistern.

Were these Medusa heads meant to somehow protect the city's water supply?

Well, it's possible.

Some believe that the strange orientation of the heads was one upside

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.

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

these venetians were able to tell him where to find the sarcophagus of the

medusa which was apparently in the basilica system so the reason that that

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.

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,

elaborate tomb for this legendary monster?

It's a fascinating claim that, at least for now, remains a mystery

carved in stone.

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're 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.

The Russian Arctic.

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 Yadam

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 Amal Peninsula in Siberia are formed in permafrost, and

they were all sort of in the neighborhood of 50 meters deep, about

deep.

Permafrost is permanently frozen ground.

It occurs in Siberia and in northern Canada and in Alaska, in the Arctic

regions. And the permafrost on the Amal Peninsula has been frozen solid for

about 40 ,000 years, at least. And they can abruptly explode.

So the sinkholes in the Amal 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, 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.

Sinkholes are a phenomenon that occurs all over the world.

There was a famous example in Guatemala in the center of a street intersection

that was nearly 300 feet deep.

And at the sudden sinkhole...

that could have made a 30 -story building disappear.

Sinkholes can be extraordinarily disruptive and dangerous.

Some can be catastrophic, where there's a sudden failure over a giant cavity,

and then a massive hole will just make things disappear.

And others are subtle, slow burns, where there are 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.

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 lived 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

many other things that we haven't even thought about yet.

So important, important discoveries are coming from cave environments and the

life within them.

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

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

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 really only scratch the

surface of our planet's deepest, darkest secrets that continue to remain.

Unexplained.