Original subtitles

WILLIAM SHATNER: Secret military bases,

built deep underground.

Intricate cave systems

that were never meant to be discovered.

And an arctic expedition to the center of the Earth.

What is it about a tunnel or a cave

that piques our interest?

Is it curiosity about what the dark

and narrow passageways may lead to?

Or could it be the inherent danger

that comes with being deep underground?

For centuries,

mankind has explored the depths of the Earth,

and found not only natural resources,

but what might be considered a perfect hiding place.

Could the stories of secret technology,

lost cities and hidden treasures lying beneath our feet be true?

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

♪ ♪

As any fan of spy movies will tell you,

it’s not difficult to believe the United States government

has a closet full of secrets.

But what could be so secret that they feel the need

to keep it hidden so far underground?

Area 51 is the world’s most secret installation.

Historically, we know that it was used

for the testing of early spy planes,

like the U-2

and the SR-71 Blackbird.

A number of whistleblowers have come forward claiming

to have seen levels, up to about seven or eight levels,

deep below the desert,

protecting whatever’s really going on at Area 51.

TRAVIS TAYLOR: If there are these large tunnels

and pathways under Area 51,

so many people would have to cooperate in building it

that it’d be difficult keeping it a secret.

But you could also ask the question,

when was the last time you paid any attention

to a new subway tunnel being constructed

under the city in which you live?

You probably never paid attention to it.

So, it could be built right under your nose,

and you would never know it or care for that matter.

SHATNER: Deep beneath these snowy peaks,

encased within thousands of tons of solid granite

lies the most sophisticated and impenetrable

military installation in the world.

The Cheyenne Mountain Complex was built in the ’60s,

and was operational in 1967,

and the idea was that you have all of your information

for our national missile defense and offense capabilities

to be operated from there.

So, it had to be able to withstand major natural

and man-made disasters, such as a nuclear attack.

So, Air Force and Army and Navy,

all of the services, they were in there.

Basically, it was the nexus

of our Cold War defense center.

REDFERN: The Cold War was at its height,

and for that reason,

it demonstrated why we had the need

to have these kind of facilities deeply below ground.

To allow them to hopefully survive a nuclear attack

if the worst scenario ever happened.

SHATNER: Originally built for a staggering cost

of almost $150 million-- or over one billion today--

the Cheyenne Mountain Complex spans an incredible five acres

and contains 13 three-story buildings

capable of housing up to 2,000 personnel.

Independent power, heating and cooling systems,

a lake that serves as a reservoir,

and 25-ton blast doors

capable of withstanding the force

of a 30-megaton nuclear explosion.

TAYLOR: If there was a nuclear attack, a direct hit,

Cheyenne Mountain was deep enough

that you would not be dry-roasted.

You would survive the heat.

Then, you got gamma rays and radioactive fallout,

but the best protection is material.

Dirt, earth, rocks.

All these things are going to protect you from that.

SHATNER: The Cheyenne Mountain Complex was so central

to the United States government’s survival plans

that they eventually realized

it may not actually be a good idea

for it to be so isolated.

REDFERN: Let’s say,

in a worst-case scenario,

that Washington, D.C. gets taken out

by a nuclear weapon.

Well, one of the theories is

that the president and all of his staff would quickly be taken

to the Denver Airport.

The Denver Airport not being too far from Cheyenne Mountain,

it would make great sense

to have a tunnel rail system deep below the nation,

which potentially could go

from Cheyenne Mountain straight through to the Denver Airport.

When they were building that airport,

there was this massive,

and I mean massive, hole in the ground.

And buildings were going up in there.

Buildings that, all of a sudden, you know,

you-you didn’t see them when the airport was finished.

And their answer to that was,

"Well, the buildings were incorrect.

"And we had such a big hole,

we just covered them over and kept building."

Who-who would buy that?

REDFERN: When the airport was being constructed in 1995,

some of the staff who were working on the facility itself,

construction staff, engineers--

they claimed to have seen

what appeared to be large tunneling devices,

and also large tunnels,

which shouldn’t really have been there.

A lot of people theorize that there is something

to this deal about Denver Airport,

because an airport is an extremely secure location

on the surface.

You can’t just walk in and go anywhere you want.

That’s good

if you’re hiding stuff underground.

SHATNER: Could the United States government

actually be operating a top secret tunnel system,

whose epicenter is beneath one

of the largest transportation hubs in the country?

According to some experts, the answer is yes.

And that may only be the tip of the proverbial iceberg.

REFERN: For strategic means, it’s believed

the United States has numerous underground facilities,

which essentially have been bored out

with these gigantic boring machines

to create tunnels

to essentially take people

all across the United States in the event of an attack.

We’re talking about trains deep under the surface

to essentially go from the East Coast

to the West Coast, to the north, to the south

to hide from the attacks that might be going on.

There has been a great advance just in civilian

and openly known abilities to dig tunnels.

This is being taken advantage of by Elon Musk

in his Hyperloop concept

that we can create long-distance

underground transportation systems.

So, we can only imagine what has been accomplished

in the secretive world of the U.S. military.

SHATNER: It’s not difficult to imagine

that in certain doomsday scenarios,

the governments of the world would prepare

fail-safe, underground tunnels.

And it appears the defense capabilities

and anonymity of moving underground

is not a modern concept.

Because there are caves

high in the Andes mountains that not only concealed

an ancient civilization but also

their hidden treasures.

SHATNER: In the eastern part of Ecuador, not far from the border of Peru,

is the entrance to a cave

known as Cueva de los Tayos,

or the Caves of Tayos.

Located at an elevation of 2,600 feet,

this winding cave system

contains a network of tunnels approximately three miles long

and is considered sacred

by the native Shuar people of the region.

But what’s most remarkable about these caves

is that somewhere within its depths

is believed to be a tomb that was constructed

by a mysterious lost civilization.

The Tayos Caves are arguably

one of the most enigmatic cave systems in the world.

They first came to note in the 1960s,

when an explorer by the name of Juan Móricz

came suddenly out of the jungle,

claiming that he’d descended hundreds of feet

into the Tayos Caves

and had seen incredible things.

This included

this tomb chamber that is supposed to contain

evidence of a lost civilization.

Most particularly, what he described

as a "metal library,"

of these metal tablets,

seemingly in gold, with these weird glyphs

written all over them,

that were carved by some kind of ancient race

that has been entirely lost to us.

And Juan Móricz claimed

that this is what he’d seen and wanted

to tell the world about this.

SHATNER: A metal library containing gold tablets?

Juan Móricz’s extraordinary account

sparked worldwide curiosity,

and many began to wonder whether there really were

ancient gold tablets hidden somewhere

in the Tayos Caves.

COLLINS: Anybody reading that story

would have been fascinated by the mystery of the Tayos Caves.

And this is, indeed, what happened,

because it attracted the attention

of a Scottish civil engineer by the name of Stan Hall,

who decided that he would create

an expedition to explore the caves.

SHATNER: In 1976, Stan Hall recruited

a number of people to join his expedition

to the remote jungle of Ecuador in search of the gold tablets,

including American astronaut Neil Armstrong,

whose participation helped to draw

worldwide interest to Hall’s efforts.

MUÑOZ: My father became very obsessed

with this treasure story.

The expedition

that he mounted is still today

one of the largest of all time, and it’s because it involved

over a hundred personnel from both the British

and Ecuadorian governments’ special forces,

and some universities sent their scientists.

The main challenge was entering the cave,

because the only way to get in

is this very deep hole in the ground.

SHATNER: According to Stan Hall’s account

of the expedition,

the entrance to the Tayos Caves was a hole

that required the group to descend one by one

more than 200 feet.

Soon after they entered the cave,

they made an extraordinary discovery.

DAVID KUSHNER: They saw these walls

leading from one chamber to the next that looked smooth,

and they looked like they’re cut at right angles.

So, it looks like the ruins of some lost civilization.

SHATNER: The massive walls did not appear to be formed naturally,

but instead, they looked like they were carved and placed,

as if they were man-made.

Stan Hall and his team believed that this was evidence

that a lost civilization had inhabited the Tayos Caves.

As they moved deeper into the caves

in search of the gold tablets,

they made another remarkable discovery

inside a chamber they referred to

as the Altar of Light.

MUÑOZ: Inside the Altar of Light,

there was a ray of sunlight that shone

straight into the entrance of that chamber,

and it made it really significant.

And within it, there’s a smaller chamber

in which my father’s expedition found the remains of a person.

COLLINS: They found this human skeleton,

and surrounding it

was pottery, um,

various shell objects and adornments.

But unfortunately, when they reached out

to touch this skeleton,

it fell apart, it just fragmented,

showing how old it was.

But subsequent research

would show that this was around 3,500 years old...

...dating back to 1500 BC.

And according to the local Shuar,

this was one of their ancestors,

the race that had inhabited the caves before them.

SHATNER: The discovery of this ancient tomb

was an astonishing and unexpected find,

one that heightened the mystery of who inhabited the Tayos Caves

and led many to wonder what else might be hidden

within this vast network of underground passages.

A few interesting things happened

during Stan Hall’s expedition.

They did not find the metal library,

but they did find other things.

Number one, it had these walls

that were cut at right angles,

and number two, they found a corpse

that was dated to go back to 1500 BC.

So, even though they didn’t find the metal library,

Stan was still convinced that it was there somewhere.

SHATNER: An Altar of Light pointing to a bejeweled ancient skeleton.

It’s certainly a remarkable image,

one that would seem like it came from the pages

of a harrowing adventure novel, and yet,

it exists deep in the Tayos Caves.

But where are the golden tablets?

Perhaps clues can be found by exploring another cave

from an ancient civilization whose main purpose

was to never be found.

SHATNER: In a small village,

a group of farmers begin draining several ponds

to create space for new planting fields.

But as the waters recede,

they make a remarkable discovery.

CLEMENTS: The villagers

saw the water was slowly going down

and there were steps going down.

So, they got very excited that this was some kind of,

you know, archaeological discovery.

And when they went inside,

they found that they were in a group

of underground man-made caverns.

SHATNER: Once the water was completely drained,

the villagers discovered a series of astonishing chambers

that had been carved underground centuries earlier.

Further excavations revealed the existence

of 24 massive caverns.

Each cavern covers an area

of approximately 11,000 square feet,

or roughly the size of two mansions.

Archaeologists have also noted that the walls, pillars

and ceilings of the caverns were all precisely carved

to have an unusually smooth and symmetrical appearance.

All told, the massive underground complex

measures 320,000 square feet,

which is the size of more than five football fields.

These incredible man-made structures

are collectively referred to

as the Longyou Caves.

The discovery of the Longyou Caves

is very fascinating

because there was absolutely nothing to suggest

that there were these carved, beautiful caves

actually beneath the water.

And so, when the actual caves were discovered,

everybody was absolutely amazed,

because there was no indication that they existed.

They have these pillars inside them

which are anything up to 100 feet in height,

and there are long corridors,

there’s some beautiful relief carvings on the walls

of animals and dragons, and things like this

that are of immense sophistication

in their design and the manner that they were carved.

NEWMAN: When you look at these carvings,

the chisel marks are so uniform,

there’s very detailed shaping of some of the stones.

To the untrained eye, it looks like something

created in the last 20 or 30 years.

Some people suggest the caves

are one of the wonders of the ancient world,

but they weren’t discovered until 1992.

So, this raises so many questions.

SHATNER: Based on the carvings on the walls

and artifacts recovered in the chambers,

archaeologists estimate

that the extraordinary Longyou Caves

are roughly 2,000 years old.

While there are similar

ancient underground structures throughout China,

the size and sophistication of the Longyou Caves is unique.

But just who exactly carved these enormous caverns,

and why, remains a mystery,

because there is no mention of this site

in the historical record.

How is it possible that the existence

of 24 caves can go unrecorded?

This is very strange because the Chinese

have always been very meticulous

in the way that they’ve recorded provincial data.

And yet, there was absolutely nothing.

No mention whatsoever of these caves.

CLEMENTS: The caves themselves

are so massive, such a huge undertaking,

it’s not something that a bunch of local villagers

might have done for a hobby.

We can estimate that it would have taken 1,000 people,

working 24 hours a day,

six years to carve out these caverns.

There ought to be some kind of geographical feature,

some kind of odd-looking hill

or an infield valley or something,

where all that sandstone has gone,

but we don’t know where that is.

SHATNER: In recent years,

some historians have suggested that the Longyou Caves

disappeared from history because they were intended

to be secret caverns

that were never meant to be found.

We do not know who built the caves.

But we can guess, considering the fact

that they’re 2,000 years old,

that they were built in a period called the Warring States.

And at the time, China was a patchwork

of different kingdoms all fighting each other.

And the fact that they are not mentioned

in ancient documents could well be

a factor of their use for some kind of strategic purpose.

So, "We’re not gonna tell the enemy

"where our secret bunkers are.

We’re not gonna tell them where our secret supplies are."

SHATNER: Could the Longyou Caves have been

a subterranean place to hide critical supplies,

or even people, from encroaching enemy forces?

Or could their purpose be tied to the deep waters

that shrouded their location?

What’s interesting about the Longyou Caves

is it seems that when they flooded,

the water helped maintain the structural integrity.

Somehow, it took the load off

some of the beams, for example, holding up the roofs.

Now that the caves have been drained,

the Chinese have been obliged to add extra buttressing

to hold them up.

And so,

I think they were

a storage facility.

Um, and I think certainly many of them were intended for water.

The area is drought prone.

So in the event of some kind of military crisis,

it would have been very handy to have a secret base

with enough water for them to live on for years.

SHATNER: Were the Longyou Caves hidden beneath a pond for centuries

because they were built to be a secret water reservoir?

Or could they have had another purpose,

that has yet to be unearthed?

For now, the answer remains lost to history.

But in modern times, the caves have become

an important source of pride for the people of Longyou,

who regularly hold festivals

in this ancient underground space,

to commemorate their heritage.

The story of the Longyou Caves is

actually very fascinating because

it would seem that by looking beneath the earth itself,

we’re obviously going to discover many, many caverns,

underground chambers, or even cities

done by human hands that have been completely lost,

and that’s a very exciting prospect.

Who could have built the Longyou Caves?

It’s a question that continues to baffle

both historians and archaeologists alike.

And the discovery of this massive underground complex

shows us just how easily entire societies can vanish

from the pages of history.

But there is another subterranean mystery,

where labyrinths of bizarre tunnels are not

confined to one specific region but instead are found

underneath an entire continent.

SHATNER: In this quiet rural town,

a Bavarian dairy farmer watches in horror

as one of her cows falls

into what appears to be a sinkhole.

But after thankfully rescuing the animal,

the farmer discovers

that her cow hadn’t stumbled into a sinkhole

but rather a mysterious subterranean ruin

known as an erdstall.

Erdstall are human-carved tunnels

that extend into the ground.

You know, they’re, they’re chiseled out of stone.

COLLINS: They are like mini labyrinths,

mini underground cities,

and the erdstalls consisted

of a series of chambers and tunnels

and stretched for about 125 feet

and was on different levels.

And archaeologists have investigated it,

and have suggested that there was activity there

as much as a thousand years ago.

SHATNER: But this isn’t the only erdstall that’s been discovered.

In fact,

similar man-made tunnels have been found hidden

all across southeastern Germany and Austria.

Many of them are found just in the middle of nowhere,

um, you know, out in somebody’s field or something like that.

Or within the cellar areas

or foundations of old houses, old structures.

Some of them may be very limited.

You go in and there’s just sort of one little chamber

or two little interconnected chambers.

And some of them extend hundreds of feet

in many different directions with,

you know, all sorts of different

little sort of side chambers and passages.

COLLINS: We don’t know who carved them.

We don’t know when they were carved.

We don’t know their original purpose.

Because of this,

the erdstall tunnels are incredibly mysterious.

SHATNER: One investigator driven to solve

the mystery of the erdstalls is Dieter Ahlborn.

He has been exploring these tunnels

for more than 20 years.

SHATNER: To date, more than 1,200 long-abandoned erdstalls

have been found beneath the German and Austrian soil.

But why would anyone go

to such lengths to carve

these bizarre underground structures in the first place?

PAUL BAHN: The erdstall, which basically means

earth stable or earth stall,

is a name given to a whole series

of many hundreds of small tunnels

that exist primarily in Bavaria and Austria,

and which are one of the great mysteries of the world,

even today.

They’ve not been known for, for very long.

Normally, if you’ve got tunnels

or caves, you will find

bones or-or artifacts or something inside them.

There is nothing

in these erdstall and that has made it

very, very difficult to know what they were for.

TREISTER: They’re obviously completely dark,

and, interestingly, you don’t see areas where there were,

like, hearths or fires.

You don’t have, you know, charcoal.

But there are also these tiny little areas carved out

for a place where like a lamp would be placed

so that people who are actually building would be able to see.

So it’s clearly-- it was very well-planned

and designed and executed, and yet,

we don’t know what they were built for.

SHATNER: If the erdstalls weren’t made

for any practical purpose, then why were they built?

Well, some people believe that they weren’t created

to be used by the living but rather

they were designed to be a resting place

for the souls of the dead.

A very unique theory, uh, about erdstalls

was that there was a lot of Christian ideas

as to their function.

And they would have been used

to collect the souls of the dead...

...where they would remain until the day of judgment

and then they would rise to go to heaven.

SHATNER: Is it possible

that the erdstalls were constructed by Christians

throughout Central Europe

to be holding chambers for people’s souls

as they waited to get into heaven?

Perhaps.

But on the other hand, there are some archaeologists

who believe that explanation isn’t quite good enough.

And one reason is that-- as it turns out--

erdstalls aren’t only being found in Germany and Austria.

People are discovering them all over Europe.

BAHN: There are similar tunnels in many parts of Europe.

Even Israel has some, but it’s very, very difficult

to see any kind of linking, uh, phenomenon between them.

Perhaps there was some widespread belief

that we know nothing about

because they were clearly of great importance.

TREISTER: Humans have been able to chisel

into stone for thousands of years.

There’s probably countless structures

throughout the ancient world that are in places

where we just don’t know that they exist.

SHATNER: Were the erdstall tunnels built to hide from one’s enemies,

or to store the souls of the dead?

The theories surrounding these narrow winding passages

are as vast and varied as the tunnels themselves.

But there’s another theory, that pushes the idea

of not just an underground society

but of an entire underground world.

SHATNER: Acclaimed novelist Jules Verne publishes

the science-fiction novel

A Journey to the Center of the Earth.

The book, which chronicles an adventure

through a fantastical subterranean world

filled with mammoth creatures, underground oceans

and crystal caves, is an instant success.

But despite Jules Verne’s insistence that the story

is nothing more than a work of fantasy,

many wondered, could some of what he wrote

actually be true?

We live on the surface of planet Earth,

but going back many thousands of years,

we have wondered what is beneath our feet.

Is it solid or is it possible that it could be hollow?

BRANDENBURG: One of the most fascinating people

in the history of science is Edmond Halley,

who the Halley’s Comet is named after.

Based on his explorations,

he discovered the Earth’s magnetic field is fluctuating,

that it was not a steady uniform object.

It was changing in time.

It appeared to be kind of rotating

relative to the surface of the Earth.

So he came up with a quite elaborate model

to try to explain what he was seeing

in his measurements of the Earth’s magnetic field.

And one of the interesting theories

he came up with was the idea of a hollow Earth.

REDFERN: Edmond Halley believed there were essentially three crusts,

three rotating crusts which surrounded

this huge, hollowed-out portion of planet Earth.

SHATNER: A hollow Earth?

Is it possible that there’s actually another world

located deep within the Earth’s crust?

Could Jules Verne’s fantastical notion of a subterranean world,

located deep underground,

have been based not on mere imagination

but on scientific fact?

Well, if one legendary explorer is to be believed,

the answer is yes.

And he claimed to have even seen it firsthand.

REDFERN: Rear Admiral Richard Byrd, in essence,

was somebody who we could equate

with like a real-life Indiana Jones.

He traveled around the world

having expeditions and adventures.

The primary reason why Admiral Byrd

was able to go on these expeditions

was because he was a very adept pilot.

He was actually given the Medal of Honor.

That demonstrates as to how revered he was

in terms of his expeditions.

He was someone who visited the polar areas,

which really fascinated him.

And in 1926,

while flying over the caps,

he was able to see what he described

as like a large green valley.

NEWMAN: One of the ideas about this whole hollow Earth theory

is that there were multiple entrances all over the world.

But most notably, the North and South Poles

are possibly the entrance for these.

REDFERN: According to the story,

Byrd described massive forest areas,

nurtured by some kind of sun

deep inside the planet itself.

And he claimed to have seen a fully-grown woolly mammoth,

which-- it would be amazing if that was true.

SHATNER: Underground rivers, extinct mammoths,

and a sun deep inside the Earth?

Could the incredible account of Admiral Byrd,

one of the world’s greatest explorers,

actually be true?

But if so, where is the evidence?

There’s water on the surface in huge oceans.

What if there’s a hole in the mantle

that allows some of it to drain somewhere?

Did it get captured there

when the Earth was created, in its formation?

I’ll give you an example.

In Mammoth Cave in Kentucky,

there were the remains of a shark

from 330 million years ago.

An extinct creature found in this cave.

In a cave-- not an ocean but in a cave.

What if this cave was part of an ocean system

that went deeper underground?

As a scientist,

the first thing I will concede

is that we do know there are vast caverns below us

and there are vast structures

that are dynamic under the earth.

And so it’s certainly possible.

The Earth is about 7,000 to 8,000 miles in diameter.

That’s pretty big.

We’ve only ever drilled to about eight miles deep.

We have no idea what’s at the center of the Earth.

You know, we have theories

that there’s this molten ball core,

iron core spinning around, it’s magnetized or whatever,

but we really don’t know.

There could be a completely different world

that we just don’t understand yet

because we’ve never been there

to do experiments and science on it

and understand what’s going on.

SHATNER: How deep will we eventually send humans

down into the mysterious depths of our planet?

Even utilizing advances in technology,

the journey through the Earth itself is difficult.

What have recent mining efforts revealed

about what lies below?

SHATNER: In this nation,

located between South Africa and Mozambique,

stands an ancient, stepped mountain canyon

known as Bomvu Ridge.

The Bomvu Ridge is significant

because according to experts,

it is the site of the oldest mine in history.

Radiocarbon dating, uh, shows evidence

that this place was mined 40,000, 50,000 years ago.

And 50,000 years later,

the place is still producing iron today.

So the history of the human species,

from the Stone Age to the Digital Age,

has this relationship to going after

the metals that we need at that particular place.

SHATNER: It’s astonishing to think that from the humble beginnings

of the first mine at Bomvu Ridge,

there are now more than 6,000 active mines

located all over the world.

Some of today’s mines extend incredibly deep into the Earth,

such as the Mponeng Gold Mine in South Africa,

which reaches an astonishing 2.5 miles underground.

The deepest gold mine is located in South Africa.

It’s about four kilometers deep.

You have to do a couple of different things

if you want to mine stuff from that far underground.

It’s one thing to drill a hole, right?

It’s another thing to drill a hole in such a way

that you can send people and equipment

safely there and back on a daily basis.

You have to be on the lookout

for evidence of minor fault lines, right?

The tiniest earthquake, the tiniest shift

can be catastrophic when you are mining at those depths.

SHATNER: Every year, more than 3.2 billion tons

of iron, gold, silver, platinum and other precious metals

are extracted from our planet.

Today, we rely on mining

perhaps even more than our ancestors did

because modern electronics depend

on what are referred to as rare earth elements.

The term rare earths refer to 17 chemically similar elements

that have fantastic magnetic and conductive properties.

They are called things like europium, lanthanum,

praseodymium, cerium, and together

they are absolutely essential for life as we know it today.

We need these elements for medical technologies,

communications, computer hardware.

But eventually rare earth elements

are going to peter out.

And the real question is,

how deep will we go into the Earth

to get the elements that we need?

I think the simple answer to that is,

we will go as far as our technologies allow us to go.

SHATNER: Will the Earth’s supply of rare metals eventually run out?

It appears the only way to find out

is to keep digging into our planet

in search of both essential resources and also...

answers to the many mysteries

that lie beneath our feet.

EDWARD GARNERO: As humans, we share

this curiosity,

this desire to understand what’s beneath our feet.

And we want to explore.

We want to follow the breadcrumbs of this mystery

of what is down there.

COLLINS: Geophysical research is revealing cave systems,

caverns, and obviously whole cities

that were never known about in the past.

And we are going to discover incredible things

beneath the earth in the future

that we could not even have imagined existing before.

NEWMAN: There are just so many of these incredible

underground cities, caverns, crystal caves,

tunnels and catacombs everywhere around the planet.

It does feel like we’ve just really

scratched the surface of what could be below us.

There could be much more.

There’s a remarkable amount of research

still to be done under the surface of the Earth.

While it’s clear that there are still many mysteries

right beneath the Earth’s surface,

maybe the next time a lake is drained,

the remnants of a lost civilization will be uncovered.

Or perhaps the discovery of an artifact

or ancient inscription will explain

why our ancestors dug elaborate tunnels deep underground.

What other subterranean hiding places

are yet to be discovered?

What secrets do they hold?

Well, for now that remains... unexplained.

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