All language subtitles for The UnXplained S02E05 The Underground World 1080p AMZN WEB-DL DDP2 0 H 264-Kitsune_track4_[eng]

SHATNER: Subterranean cities,

hidden from human eyes.

Mysterious tombs,

where mourners could speak
directly to the dead.

And caves filled
with giant crystals

that are as beautiful
as they are deadly.

For centuries,
mankind has wondered,

what lies deep beneath our feet?

Is there a subterranean world
filled with mystery and danger?

Or one that hides untold riches?

Perhaps even the secrets
of the universe?

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

♪

Bordered by the Black Sea
to the north

and by the Taurus Mountains
to the south,

lies the craggy regions
of Cappadocia.

Here, wind, water, and time
have sculpted the volcanic rocks

into bizarre
and almost surreal shapes.

But believe it or not,
the most unusual thing

that was ever discovered
in this otherworldly landscape

was found not above ground,
but far below.

In 1963, during a simple
home renovation

in the town of Derinkuyu,

a cave wall was punctured,
revealing a passageway

to what appeared to be
an intricate underground city,

thousands of years old
and more than 280 feet deep.

Derinkuyu is
a pretty amazing story,

'cause just imagine
that you're working

in your basement one day.

You knock a hole in the wall,

and what you find
is a city that is,

it's sort of the size
of a-a really large castle,

but it's actually underground.

Uh, and it was done in a time

before there was really
any mechanical means

to provide water or ventilation,

or any of those things.

ANDREW COLLINS:
The only thing on the surface that you can see

are what appear to be wells.

As you approach Derinkuyu,

you would certainly not be aware

that you were walking
over the position

of an underground city.

It would have been the home
for thousands of people

at a time at some point
in the distant past.

SHATNER:
Home to thousands?

It seems hard to believe
that an entire city

could actually
exist underground.

And yet, experts claim
that Derinkuyu housed

an estimated 20,000 men,
women, and children

and included religious centers,
storerooms, and living rooms.

But how?

MICHAEL DENNIN: Derinkuyu
appears to be a combination of

leveraging natural cave systems,

and then building in
human technology to connect it.

And one of the things
that's fascinating about this

is the ventilation system,

because here you're really just
worried about getting air in,

and waste like CO2 and air out.

You're gonna be able
to store water.

If you think about
how much fresh water

is already
naturally underground,

we're constantly digging down
to get to wells,

but if you're
already underground,

the wells are right there.

So you have the possibility
of great fresh water sources.

SHATNER:
Many archeologists and scholars

claim that Derinkuyu
was most likely built

around 800 BC by the Phrygians,

a Bronze Age people
related to the Trojans,

and was intended to serve
as a temporary shelter

from invasion.

But others believe it was built
by the Hittites,

a warrior people
mentioned in the Bible,

who flourished
a few hundred years before that.

BURROWS:
This is in a place that's had multiple wars.

There's conflict taking place
all the time.

And when you're above ground,

people can see you for miles.

You know, each night
you're lighting fires

to get some light
inside the space.

And so, that can be seen
from 20 miles or more.

Once they go below ground,
they're invisible.

JOHN BRANDENBERG:
It has, uh, massive stones that can be lowered

to block passageways
that human beings

can only go through
in single file,

making it very easy to defend.

There are also rumors
that it is connected

by a vast tunnel system
going many, many kilometers

to other underground cities
in the neighborhood.

So, it had emergency
escape routes.

So the whole thing
was a quite remarkable

subterranean fortress.

SHATNER:
While there is evidence to suggest

that several different cultures
have used Derinkuyu

as a fortified sanctuary

in the centuries
that have passed

since its initial construction,

some researchers have theorized
that it is much, much older

than mainstream
archeologists think.

And that its original architects

had an even bigger threat
in mind when they built it.

COLLINS:
The curator of Derinkuyu in Cappadocia,

Omer Demir, had found
the evidence suggesting

that each of the different
stories or levels

would seem
to have been constructed

by different cultures
at different times.

And what he found
was incredible.

For he discovered
that there were tools,

stone tools here,
that dated back

to Paleolithic times.

The implications
of this are enormous,

because what this suggests
is that Derinkuyu goes back

at least 12,000 years,

and was present at the end
of the last Ice Age;

and may well have been a place
where our ancestors

sought refuge at the time
of a cataclysm,

a cataclysm that may well
have devastated

large parts of the Earth

at the very end
of the Paleolithic era.

SHATNER:
A cataclysm?

Could it be that
the original inhabitants

of Derinkuyu built it
as a shelter to protect them

during the Earth's
last great Ice Age?

A time when the Earth's climate
was estimated to be

more than 10 degrees colder
than it is now?

So cold that hardly anything
could grow,

or even survive, above ground?

DENNIN:
When you build your city underground,

you get two major
off-the-top advantages,

one sort of environmental.

You're gonna have
great thermal properties.

It's gonna be cool
when you need it to be cool,

but it won't get too cold,

so it'll be kind of warm enough
when you need it to be warm.

Thinking about when this
was built and inhabited

really remains this exciting,
mysterious, open question.

Even this idea of
other cities underground,

like, how common was that

for humans to build
their homes underground,

versus on the surface
of the ground?

Derinkuyu was rediscovered

during the 1960s by accident.

And there are still areas of it

that have not been explored
to this day.

So the implications of this
are incredible.

It does feel like
we've just really

scratched the surface
of what could be below us.

There could be much more.

There could be many more of
these cave systems in Turkey.

So there's mysteries
still to be found

deep within the Earth.

SHATNER:
The truth is, we may never know for sure why,

or even how these ancient
underground cities were built.

And that's especially true
in the case of Derinkuyu;

because the secrets
of its construction techniques

seem to have disappeared,
along with whoever built it.

BURROWS:
Even today, with all of the modern technology

that we have, it would be
incredibly complicated

and a massive undertaking
to do something like that.

And sometimes, we look at these
ancient marvels,

and we try and figure out,
could we build that today,

to that level of accuracy,

that sort of quality
of construction

in that amount of time?

And the answer's no,
which is pretty amazing

that several thousand years
later, they're better than us.

SHATNER:
Is it possible the builders of Derinkuyu

knew something we don't?

Not merely about
constructing cities

deep within the Earth,

but about humanity's need
for such refuges

in order to survive?

It's a fascinating, and perhaps,
slightly unnerving prospect.

Not unlike another
underground structure,

one that exists in order
to maintain communication

after the end
of civilization itself.

NICK REDFERN:
In the event of an attack, it's believed the United States

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,
um, deep below the desert,

protecting whatever's
really going on at Area 51.

TAYLOR:
If there are these large tunnels and pathways under Area 51,

so many people would have
to cooperate in building it

that'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, uh, 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, uh, 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, the 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 gonna
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.

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."

W-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.

REDFERN:
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

and 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:
But as profound and disturbing as is the notion

of a vast network of underground
bases and secret tunnels,

there are some underground
worlds that are formed

not by man but by nature.

And in at least one case,

the formation is so beautiful

that it's easy to forget
that, just like mankind,

Mother Nature
can also be deadly.

TAYLOR:
FIn the mineral-rich towne gotten aof Naica,they are,

more than 1,000 feet below
the surface of the Earth,

a group of miners drilling
through a cave wall

make a remarkable discovery.

Not of precious metals,

but something far more wonderous
and mysterious.

A glittering cavern, untouched
for half a million years,

filled with the largest crystals
ever found.

TAYLOR:
It's amazing.

There are crystals
that are huge.

So big you can actually walk
across them.

And for these crystals to have
gotten as big as they are,

they had to have been
growing for hundreds

of thousands of years.

It's a fascinating location.

This is something you would
think would only exist

in the world of science fiction
and fantasy,

but the fact is,
it's all too real.

SHATNER:
Measuring up to an astonishing 40 feet in length

and weighing as much as 55 tons,
the giant beams

are the purest form of selenite
crystal in the world.

Crystals are the most
orderly structure in nature,

and they have the ability
to absorb frequency and energy

from objects around them.

Now, it is my theory that these
giant pylons have the ability

to transmit and receive
information and light.

SHATNER:
Only a few months after its discovery, a team of scientists

was sent in to study
this geological anomaly.

They soon learned
that Naica's Crystal Cave may be

not only beautiful but deadly.

TAYLOR:
It's 90% to 100% relative humidity inside this cave.

And it's always over
113 degrees Fahrenheit.

You might think, "Well,
113 degrees isn't that bad.

"It gets 140 degrees

in the deserts in Iraq all
the time and people live there,"

but the relative humidity
in those deserts is 10%.

In this cave,
there's so much humidity,

you can't sweat
and cool your body off.

HUTCHISON:
I received an invitation to go

on an expedition and exploration
of these giant crystals.

I was aware
of the heat and humidity,

but nothing prepared me

for what hit me
when I entered those caves.

I felt like I was hit
by a tsunami

of tremendous heat and humidity.

I started shaking, I was scared.

These are near-fatal conditions.

SHATNER:
Due to the Crystal Cave's lethal atmosphere,

the research team was restricted

to ten-minute exposures
to avoid being cooked alive.

For gemologist Leela Hutchinson,
the most unexpected discovery

wasn't something she saw
but something she heard.

HUTCHISON:
One of the things that I realized as I had to focus

so intensely
to keep my wits about me,

was this ability
to hear a low hum.

[low hum sounding]

And I thought
that was very strange.

I didn't know
if it was the machinery

that were in the tunnels

or if it was something that
might be in the cave itself.

And later I realized,
could this have been

extremely low-frequency
resonating in there?

Nikola Tesla was quoted
as once saying

that "we may not understand
the growth of a crystal,

but nonetheless,
it is a living being."

SHATNER:
Energy from living, giant crystals?

In an attempt to navigate
the unusual properties

of the crystals
within the Naica cave,

NASA developed
special cooling equipment

allowing scientists to withstand
the cave's lethal conditions

for as much as 30 minutes
at a time.

And according to their research,
the hum emanating

from the Crystal Cave
may not be because its crystals

are storing energy,
but because they're harboring

something even more unexpected:
life.

HUTCHINSON:
In the 2008 expedition, the NASA scientists

went into the caves
and drew out fluid inclusions

in the largest crystals,

and found microbes that are
considered to be extremophiles.

Microbes that are
living organisms.

They're single-celled,
and they have the ability

to survive and thrive
in extreme environments.

One of the interesting things
about the extremophiles was

there is nothing
in our genetic database

that comes close to what
these extremophiles are.

SHATNER:
Microbes unlike anything seen on Earth.

NASA's astrobiology report
suggests these extremophiles

are up to 50,000 years old,

and could survive
on other planets.

But many wonder: is there a risk
involved in handling

these never-before-seen
organisms?

The Cave of the Crystals
can be a dangerous place.

For example,
not long after it was found,

one of the workers actually
snuck back into the cave.

Not surprisingly,
he passed out and died.

Now, it's unclear if he died
from the air quality,

the heat, the humidity.

Or could it have been the
extremophile microorganisms

that were in there?

Will we ever find out
whether the giant crystals

in the Naica cave
hold the key to understanding

the origins of life?

Or is this one underground
mystery that is simply

too dangerous for us
to investigate?

Perhaps.

But not all underground worlds
are meant to help us understand

the mysteries of life.

Some actually appear
to be constructed

to bring us closer...

to what happens to us
after we die.

SHATNER:
While excavating an areaLINDA ENEIX: It's extraordinary.

for a new housing development

just off the eastern coast of
this tiny Mediterranean island,

construction workers encounter
something unexpected

and quite unusual
beneath their work site.

A prehistoric structure,
hewn from solid limestone,

dating back more than
5,000 years.

Upon further inspection,
archaeologists soon realize

that what the unwitting workers
have unearthed

isn't merely a lost
underground temple

or ancient sanctuary,

but a massive subterranean
necropolis.

The Hypogeum of Hal Saflieni.

The Malta Hypogeum is
an architectural wonder

of the ancient world.

It was carved using primarily
stone and bone tools

by an ancient population
that lived on Malta

thousands of years ago.

So, it's actually older than the
pyramids of Egypt or Stonehenge.

"Hypogeum" is basically
a word that comes

from the Greek terms for
"cavity" or "space underground."

It extends
on three different stories

about seven-point-some meters
below ground.

It covers an area
of about 5,400 square feet.

Malta's Hypogeum is, you know,
one of those great examples

of, uh, beautiful geometry,
uh, underground.

So, the geometry
isn't by accident.

It was done by somebody
who knows what they were doing.

And it looks to me, like,

you know, this was done
for a purpose.

SHATNER:
A purpose?

Although no one knows
for certain who originally built

the Malta Hypogeum or why,

the bones of more
than 7,000 souls

line its intricate labyrinth
of corridors and alcoves.

So, according to logic,

it is exactly what it appears
to be: an ancient burial site.

But logic alone
may not be at play here.

There is a very intriguing room
in the Hypogeum,

which is in
the middle level, uh,

that is called the Oracle Room.

And this Oracle Room is
basically a resonance chamber.

And it's designed,
so it would seem,

to amplify
any acoustic sound waves.

So, even by whispering,
for example,

uh, the design of the room
amplifies this

and-and makes it
into a very loud baritone sound.

KREISBERG:
The Oracle chamber itself

appears to have been
carved specifically

to allow sound frequencies
to travel,

and the ceiling
of the Oracle chamber

actually appears to have been
carved to direct sound.

There are also niches
that have been carved

in the sides of
the Oracle chamber that seem

to be a way to produce echoes
and were also created,

in a way, to tune it
to the right frequency.

So, there was
a sophistication there

that, you know, is unexpected.

SHATNER:
In 2014, while conducting a series of experiments

in an attempt to understand
the design elements

behind the Hypogeum's
unusual acoustics,

scientists discovered

an unexpected
and hair-raising phenomenon.

KREISBERG:
We set up microphones and we used recording devices

that were very sensitive,
and we carried out experiments

using the human voice in that
110 to 112, uh, hertz range.

So, these are very low,
guttural, bass sounds,

and they reverberate in a very
strong way within these temples.

[low-pitched,
resonant vocalizations]

ENEIX:
I-It's extraordinary.

It's like being
inside of a bell.

And those vibrations
are felt inside your body.

You feel it in your tissue,
you feel it in your bones.

It can raise the hair
off the back of your neck.

Imagine going there to this dark
and spooky underground place

5,000 years ago,
with this weird, eerie sound

going on around you and-and
knowing that there are bones

and-- the whole thing
is creepy now.

PAUL DEVEREUX:
It clearly was-- some sort of ritual activity went on there.

There is a niche
in the Oracle Room,

that if you spoke into it,
it could resonate

throughout the whole structure.

And so, some thought
it was significant,

that a priest might've
intoned into it a voice--

that his booming voice would go
through the whole Hypogeum.

Uh, like a god.

SHATNER:
The voice of a god?

Those who have visited
the Malta Hypogeum

describe it as feeling
like a descent

into the underworld.

As a result,
some experts believe

this subterranean nightmare
was intentionally constructed

to enable its visitors
to speak with the dead.

STEAVU:
Overtone chanting is a common practice

in many cultures
in, uh, East Asia,

and, uh, North Asia
and South Asia as well.

You do have this strong
connection between chanting

and communication with
the divine or with the deceased.

And I believe at the Hypogeum,
we may have a similar situation

in which there seems to have
been an active ritual function

of, uh, not only placing
the deceased or remains there,

but where people would attempt
to establish communication

with the dead.

In the range of 110 to 112 hertz
is a frequency that, uh,

has a specific effect
on our brain.

It puts us in a sleep state and
has us producing alpha waves.

What happens when the alpha
waves are produced

is that it kind of quiets the,
um, language center of our brain

and puts us in more of a
meditative or trance-like state.

It's similar, in many regards,
to what happens, uh,

when a shaman uses
the practice of ingesting

psychoactive chemicals,
whether it's ayahuasca

or whether it's, uh,
mushrooms of some type.

STEAVU:
So, it stands to reason that, potentially,

people could attain a different
level of consciousness

in which they would be
susceptible to have experiences

of communicating with the dead
or with the divine.

SHATNER:
As far-fetched as the notion

of communicating with the dead
may seem to us,

some believe whoever built
the Hypogeum

may have been on to something.

Because it appears
they left behind instructions.

DEVEREUX:
There is this spiralic painting

on the ceiling
of the, uh, Oracle Room.

And it's not unusual to use
spiralic shapes in prehistory

and early times to represent--
visually, graphically--

represent sound
in terms of spirals.

So it was clearly
an intellectual connection.

KREISBERG:
The clues may lie right before our eyes.

Those sounds, those songs,
those tones, those frequencies

that were played and heard

were of the utmost
importance to them.

We don't understand exactly why.

But by studying them, hopefully,
we can shed some light

on not only what
they were trying to do

but how we could use
those same concepts,

those same understandings,

to perhaps benefit
our society today.

An ancient necropolis

constructed and designed

to allow people
to send messages

into the afterlife?

Fascinating...

albeit a disturbing concept.

Not unlike one of the most
persistent and bizarre theories

about our planet.

The notion that it could be...

hollow.

SHATNER:
Acclaimed novelist whJules Verne publishest.

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 Halley's Comet
is named after.

Based on his explorations,

he discovered the Earth's
magnetic field was 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.

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, uh,

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's 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 in an ocean but in a cave.

What if this cave was
part of an ocean system

that went deeper underground?

BRANDENBURG:
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:
If another world does exist deep beneath our own,

is it possible we might be
able to one day access it?

There are those who believe
that not only is the answer yes,

but that humanity's
very future may lie

not above ground but down below.

As proof,
they point to a village in China

where thousands of people

are already living
deep underground.

in the rural region of Sanmenxia

lies an ancient network
of villages

that exists entirely
underground.

Millions of people have
lived and worked here

spending their entire lives

beneath the surface
for generations.

And not just
thousands of years ago,

in the ancient past...

but to this very day.

COLLINS:
Over the past 4,000 years

there were tens,

if not hundreds
of thousands of people

living within these underground

dwellings
in the northern part of China.

And these places,
which are known generally

as yaodongs,

continue to be built
even to this day,

and, in fact,
the modern examples

created over
the last hundred years

have got bathrooms,
they've got electricity.

If you could imagine a sort of

modern-day housing development
anywhere in the world,

everybody had a basement and
you took all of the houses away,

and all you were left with
was the basements...

that's what
Sanmenxia looks like.

It's these sort of rectangular

holes in the ground,
like a chessboard,

all over the place.

SHATNER:
There are many living options

in the underground dwellings
of Sanmenxia,

ranging from simple
one-bedroom caves...

to more elaborate
three-bedroom homes

with modern appliances
and plumbing.

But why would anyone choose
to live below ground

in a country with several
thriving cities to choose from?

One of the benefits
of living underground,

especially in very hot
and cold climates,

is below the earth, the
temperature is more moderated,

so you don't get
the extremes so much.

You know,
it's probably pretty cool

most of the time and so,

you know, they're
not bad places to be.

COLLINS:
It could well be that with the expansion

of population
at the rate that it is today

that going underground

is something that we should
look towards in the future

to solving the problems
of housing

and creating new cities.

You have to wonder,
is the day gonna come

when the Earth itself,

at least from the perspective
of the surface,

that the Earth simply will not

be able to stand
any more people.

So what's the answer?

Maybe the answer is
that we all move underground

or possibly
a significant percentage

of the population
choose to go underground.

Given everything going on
in the world today,

will we live to see the day
when our civilization

goes back underground?

It certainly seems safer.

And if that comes to pass,

will we then learn
about all the other secrets

the Earth is hiding
beneath its surface?

It seems that the answer
will be one

that for now remains buried,

and... unexplained.