[Graham] What I'm suggesting is that something that we would recognize
as an advanced civilization existed during the Ice Age.
I'm not saying that they flew to the moon.
But what I'm saying is that they were far, far, far more advanced
in terms of scientific knowledge.
Knowledge of the Earth, knowledge of the universe,
than we're taught.
And capable of astounding feats of engineering,
like the site I'm heading to in Turkey's Cappadocia region.
There's an ancient survival bunker deep underground...
built not just to shelter a few people, but to shelter thousands.
Mainstream historians have long debated its purpose,
but I believe it might just explain
how this lost ancient civilization I've been looking for became lost.
This is the story of Derinkuyu.
I've come to a region of Turkey known as Cappadocia.
About 240 miles northwest of those mysterious enclosures of Göbekli Tepe...
that were intentionally buried,
memorializing a time of great cataclysms at the end of the Ice Age.
Millions of years ago, this region's landscape was transformed
by a series of volcanic eruptions, leaving layer upon layer of ash,
which compressed over millennia into a soft stone called tuff.
A stone easily shaped by the elements,
creating what locals call "fairy chimneys."
But a stone that also allowed the construction
of one of the most remarkable large-scale projects
humanity has ever embarked upon deep beneath this soft rock.
Just a few miles from where I stand,
the mysterious hidden city created thousands of years ago has been revealed.
It's the work of an as-yet unidentified civilization
that was clearly motivated by fear.
The big question therefore is, fear of what?
The closest town, Derinkuyu, seems unremarkable.
But in 1963, developers renovating a house here knocked through a floor,
only to discover a deep tunnel, one that led to a forgotten world.
It's hard to believe, as you walk the dusty streets of this small town,
that carved out of the living bedrock right beneath my feet
is an ancient, immense and mysterious subterranean complex.
If you're claustrophobic, consider yourself warned.
We're headed underground.
This is the underground city of Derinkuyu.
A series of stone tunnels and chambers
plunging as deep as 85 meters below the surface...
creating 18 levels of rooms and tunnels.
The entire complex was hacked out of the rock with hand axes.
A disorientating warren
that occasionally widens into large open spaces.
From inside, it's virtually impossible to get a sense of the scale of the place.
But most of this underground city has been mapped,
and if we take away the rock between the spaces,
we can see a cross section of the city,
and it's utterly astounding.
It's an ant farm built human size, with subterranean caves and tunnels
extending over an area of four square kilometers.
To keep it well-ventilated, Derinkuyu has upwards of 15,000 air ducts
connecting the upper levels to the surface...
and more than 50 vertical shafts,
some stretching all the way down to the water table 85 meters below the surface...
giving the complex its name.
Derinkuyu means, "deep well."
Stunning in its ingenuity and architectural complexity,
it's calculated that Derinkuyu could shelter up to 20,000 people.
But questions remain.
Which people, when and why?
It's hard to know precisely,
because Derinkuyu is like a crime scene that's been trampled on for generations.
Many cultures have passed through this part of Turkey.
For centuries, Cappadocia occupied a place of strategic importance,
along the legendary Silk Road that connected Asia to Europe,
going back to the time of Alexander the Great.
Turkish scholar, Sevim Tunçdemir,
is an expert on the Derinkuyu tunnels and their various occupants.
[in Turkish] This region has been home to many civilizations.
If we count them starting from the beginning,
there were the Hittites,
then there were the Phrygians, then the Persians,
the Cappadocian kingdom,
the Roman Empire.
All the people who passed through used them.
They were even in use up until the Ottoman period.
[Graham] When the tunnels were first discovered,
archaeologists found artifacts left by early Christians,
and in the deepest levels, secret meeting rooms
carved out with vaulted ceilings like churches.
So the original theory,
which many historians still cling to today,
was that the tunnels beneath Derinkuyu
were carved out by Christians in the 7th century AD,
trying to hide from Arab raiding parties.
It's a tale that appeals to Western tourists.
Also, it's totally wrong.
Later excavations found evidence of people using this underground city
as early as the 8th century BC,
hundreds of years before the Christians were here.
We encounter this again and again on archaeological sites around the world.
There's a big notice based on the received wisdom of archaeologists,
and again and again that notice is wrong.
Factually wrong.
Proved to be wrong by later excavations and yet not changed.
Don't trust the noticeboards. Do the legwork yourself.
Don't rely on the so-called experts.
There were many cultures who used these tunnels over centuries,
but what I want to know is who began this remarkable project.
How far back does it go?
Historian Hüsam Süleymangil has been investigating this site for decades,
trying to unlock its mysterious origins.
So, when I look at Derinkuyu and the complexity of it, I am mystified.
When do you think that this project started?
There are several different theories about it
and none of them is really proven by science.
Right.
They couldn't find any written information.
They couldn't find any organic material to use to carbon date.
Are there any carbon dates at all?
As far as I know, there is no carbon dating.
That's extraordinary.
So, the date is still a big mystery.
I have my own theories,
but Hüsam proposes a date
based on the oldest known culture to use these caves.
Most plausible theory, according to my mind,
is actually about the 8th century BC.
[Graham] At that time, this part of the world
was inhabited by a people known as the Phrygians,
who were under threat from another empire, the Assyrians.
We know that there was a big Assyrian army coming from southeast.
[Graham] The Phrygians would've viewed the invading army with sheer terror.
The Assyrians were notorious for skinning prisoners,
impaling them and burning children alive.
According to later accounts,
when the Assyrian invaders marched against the people in this valley,
they were surprised by the defenders' innovative tactics.
[Hüsam] The Phrygians were fighting against the Assyrian army
in the guerilla type of warfare,
attacking the army in sort of unexpected places.
When the army start to chase them,
they would come and use these as hideout places.
[Graham] It's a time-honored strategy in guerrilla warfare.
Similar tunnels were dug out by the Viet Cong in South Vietnam.
And Afghan rebels did the same
to hide from superior Soviet and then American invading forces.
This official position,
that the Phrygians dug out these tunnels as a secret military base,
seems to be supported by one of the complex's more clever features...
massive stone discs that can be rolled into place,
sealing up the passageways.
These rolling doors have a diameter of up to five feet
and can weigh up to half a ton.
On the inside surface, the disc has a hole.
Insert a smaller locking stone as a handle and you can roll the door shut,
creating a smooth, formidable barrier for anyone on the other side.
But did the Phrygians really make these ingenious doors,
or were they already here?
They're carved from the same compressed ash
that made the fairy chimneys.
The same soft rock that allowed someone to dig all this out in the first place.
They're certainly sealable doors, separating the levels from each other,
requiring sophisticated engineering to fit in place.
But the stone is soft
and determined attackers armed with sledgehammers and chisels
could easily have broken through,
rendering all the effort to make these megaliths completely useless.
I'm beginning to wonder if they were designed to deter human attackers at all.
To me, these look less like defensive fortifications
and more like a clever way to allow privacy between sections,
or to prevent fires from spreading.
Even if this place was originally built as a military installation,
why dig it out here?
There was nothing here to defend.
[Sevim, in Turkish] There were no settlements
above ground in Derinkuyu until 1830.
The notion that these are places where people went to hide from invading armies
makes very little sense to me.
When an invading army comes into a territory,
they come to take, possess and occupy that territory.
All they have to do is block the entrances and wait till you die of starvation.
So that idea just doesn't make any sense at all.
See, in my opinion, I think scholarship is going too far
to say this was the 8th century BC
because those are the earliest dates that we find people using it.
Right.
But that we don't necessarily know that they made it then.
Maybe it was already made.
Quite possible, as it's still a mystery.
- It's just the most plausible theory... - Yes.
...not the only correct theory.
[Graham] The dating of this underground city
is about as insecure as it's possible for archaeological dating to be.
All the proposed dates derive from use of the structure at different periods.
The fact that I live in a house today
doesn't mean it was built immediately before I moved in.
It's a notion that leads me to question the official dating of these tunnels.
Could they be older,
much older, than the accepted theory says?
And was the reason for their construction not to hide from an invading army
but to hide from something else?
In the most ancient levels, the ones closest to the surface,
the chambers seem not to be designed for defense
but for everyday living.
[in Turkish] The underground cities were organized to cater for human living.
In other words, for daily life.
And for this, all the resources were available.
Starting with the kitchen, to the pantry, to the living spaces.
[in English] There are sections we know that they would use as cooking
and we know that they created small chimneys.
Mmm-hmm.
At the entrance floors, there are some rooms that's named as stables.
Animals would be the most valuable belongings of those people.
[Graham] One area has even been identified as an ancient winery
where grapes were crushed.
Indeed, the tunnels would make a great wine cellar.
The temperature stays quite comfortable,
no matter how hot or cold it gets on the surface.
Food would've stayed fresh longer down here.
It's clear that Derinkuyu was created to be used by a substantial population.
An underground bunker.
Think about all the modern examples
where humans have created vast underground living spaces.
The Cheyenne Mountain Complex in Colorado.
The Dixia Cheng network of tunnels dug out beneath Beijing.
The Presidential Emergency Operations Center under the White House.
None of these were built to defend against invading armies.
They were built as places of refuge,
to preserve life in the event of some kind of threat from above.
I think that's exactly what's going on...
at Derinkuyu.
But I also believe this bunker was created much longer ago
than archaeologists are willing to consider.
There is evidence that suggests
that Derinkuyu may go back as far as the last Ice Age.
It concerns those telltale marks left by hand axes
on the walls of Derinkuyu.
Just a mile outside Derinkuyu,
Turkish archaeologists exploring an ancient riverbed
found several hand axes and stone tools
dating back to around 9500 BC,
the end of the last Ice Age.
The same sort of tools
that shaped Derinkuyu's shallowest and oldest chambers.
Think about the semi-subterranean chamber at Karahan Tepe,
less than 300 miles from here,
with its columns that closely resemble
the natural fairy chimney formations of Cappadocia.
Since no one disputes
that this chamber was carved out at the end of the last Ice Age,
there's no reason why Derinkuyu
couldn't have been made at around the same time.
As Derinkuyu reveals its depth and complexity,
what's hard to ignore is the vast scale of the enterprise
and the enormous effort involved in tunneling it out in the first place.
What makes this feat of engineering even more remarkable
is that this isn't the only underground city in Cappadocia.
In 2013, construction workers stumbled across
another series of tunnels at Nevşehir,
17 miles away.
What they found was an underground city that's even larger than Derinkuyu.
Since then, more underground cities have been discovered.
The stunning truth is here in Cappadocia,
archaeologists have identified 36 such underground cities.
And if we count the ones with just two levels,
that number balloons to 200.
One of these underground cities, just five miles away from Derinkuyu,
holds an incredible secret.
The tunnels at Kaymakli run eight stories deep
over an area wider than Derinkuyu.
Based on the number of storage chambers carved into the walls,
archaeologists think this bunker could've supported up to 3,500 people.
But Kaymakli's most mind-blowing feature can be found here on the third level down.
An underground tunnel that runs far into the distance.
It's impassable today,
but this blocked tunnel is claimed by some to connect Kaymakli to Derinkuyu.
A straight-line distance of about five miles.
In fact, archaeologists have confirmed
that at least six other underground complexes in Cappadocia
are connected to one another by similar underground passages.
To my thinking, this changes everything.
These underground labyrinths aren't just isolated, individual bunkers.
They're part of a massive and widespread project
involving dozens of similar sites scattered across the region.
Only a highly motivated culture would have undertaken such a task.
What kind of threat would have been so devastating
that it could compel an entire people,
possibly hundreds of thousands of them, to carve out a new life underground?
There has to be another explanation for why these places were made
which makes better sense than hiding from invading armies.
The answer might lie in one of the region's very oldest myths,
one that dates back thousands of years to the Zoroastrians.
[folk music playing]
These are some of Cappadocia's famous whirling dervishes.
Devout followers of a religion called Sufism.
One of the few remnants of the ancient culture of Zoroastrianism.
The ancient Persian prophet, Zoroaster, founded what's claimed by some
to be the world's oldest continuously practiced religion.
Its sacred texts refer to an underground city just like Derinkuyu,
telling us exactly why it was made and by whom.
Zoroaster spoke of the first king and founder of civilization,
a man named Yima.
One day, as Yima was beside a river,
the great god Ahura Mazda appeared to him with an ominous warning.
Not of a flood, but of a fatal winter.
And he told Yima to build a vara, an immense underground shelter.
Into it, he must bring the best of men and women, and animals,
two of every kind.
Yima must store seeds of every tree and fruit,
creating an inexhaustible supply until the fatal winter had passed.
Oh, and the sacred texts also tell us
that the onset of this fatal winter would be heralded by a serpent in the sky.
The supposedly mythical vara
sounds a lot like the underground cities we find here in Cappadocia.
But mainstream historians refuse to see the connection.
The Yima myth is just another one of those myths
that archaeology assumes don't mean anything,
and yet it speaks of a terrible freezing winter descending.
[wind howling]
And just as geologists have confirmed
that there was a period of great floods during the Younger Dryas,
much resembling those described in myths,
they've also noted that afterward, temperatures around the planet plummeted.
A fatal winter indeed.
And then there's that detail linking the onset of the fatal winter
to the arrival of a great snake out of the sky,
just like the snakes we've encountered in the myths of the ancient Aztecs,
or of the Iroquois.
Serpents always associated with cataclysms.
I'm also reminded of those pillars at Göbekli Tepe
covered in carved depictions of snakes seemingly raining down from the sky.
Or of the snake etched into Malta's great temple of Ġgantija.
Or of Serpent Mound in North America.
Of course, archaeologists don't connect these ancient symbols and traditions
from opposite sides of the planet to one another at all,
let alone to a singular event.
But we've now seen how different and supposedly unrelated structures
all around the world
seem to have benefitted from a legacy of very ancient knowledge.
A shared legacy of unknown origin.
The stunning implication is that during the Ice Age,
an advanced civilization, whose influence spanned the globe,
coexisted with the hunter-gatherers who we know were also present at that time.
A civilization that was destroyed
in the mysterious cataclysms of the Younger Dryas.
Could all these references to serpents also be part of that legacy?
A warning left behind by survivors.
What it comes down to, for me, is that we humans are a species with amnesia.
So badly knocked on the head by the cataclysms that occurred
at the end of the last Ice Age,
that we've forgotten an important chapter of our own story.
And that can be a big problem because as with so many of these myths,
a later story about Yima ends with a clear warning from the gods
that one day, a similar catastrophe would return.
[thunderclap]
Could it?
For a long time, it remained a mystery as to what triggered the floods,
fires and plunging temperatures of the Younger Dryas,
but new geological evidence has suggested a terrible possibility.
Evidence still visible today
in the scarred landscape of prehistoric America,
where I'm headed next.
I'm quite persuaded that the origin of serpent symbolism
has to do precisely with those serpents in the sky
that we call comets.