All language subtitles for Secrets.in.the.Sand.S01E06.1080p.WEB.h264-BAWD_track3_[eng]

NARRATOR: A huge discovery
in South America

raises questions about early
human history in the region.

- Why would they come here?

With the dry conditions, limited
resources and low oxygen,

it's not exactly
a welcoming environment.

NARRATOR: A terrifying discovery
deep within a Saudi Arabian cave

leaves experts searching
for answers.

- Could they be here as victims or
were they the predator themselves?

NARRATOR: And a haunting audio
phenomenon found in the Gobi Desert

baffles scientists.

- It's almost unbelievable
that it's actually happening.

But where could the sound
be coming from?

NARRATOR: Astonishing discoveries

unearthed from the depths
of the desert.

Ancient lost cities.

Forgotten treasures.

Mysterious structures.

Extraordinary curiosities
once lost to the sands of time

are finally revealed.

As new technology uncovers
remarkable tales

hidden beneath the deserts
of the world,

the Secrets In The Sand
will finally... be exposed.

In the Puno district of
southern Peru,

high up in the Andes Mountains,

is Lake Titicaca.

Covering around
8,300 square kilometres,

this immense lake is the largest
body of freshwater in South America.

- The lake and basin are thought
to have been created

millions of years ago

when a huge earthquake
rattled this part of the Andes.

And it sits at an astounding
3,800 metres above sea level.

- The highlands in the Lake Titicaca
basin are really inhospitable.

It's cold and dry,

and because of the altitude,
there isn't much oxygen in the air.

- It's a largely treeless,
resource-thin environment.

But despite this, the area has
a rich human history

dating back thousands of years.

NARRATOR: Around 30 kilometres
west of the Lake,

a local man discovered
a scattering of artifacts

buried in the sandy ground.

- He keeps coming across ancient
projectile points on his land,

so he asks archaeologists working
nearby to come have a look.

HARIDY: Excavating an area of around
37 square metres

and about 30 cm deep,
reveals dark stains

where the earth has been
blackened by ash from fires.

- This indicates human presence,
from hundreds,

possibly even thousands
of years ago.

NARRATOR: The discovery
was made near Mulla Fasiri,

a community of Aymara people.

The Aymara are a large
indigenous group,

who've lived in this region since
before the Inca took control

in the 15th century CE.

- Nowadays, the Aymara number
around three million people

spread out across Peru, Bolivia,
Argentina and Chile.

They're mostly herders and farmers,

despite living in harsh environments
high up in the Andes.

ELLIS: Their lands became a vital
part of the Incan empire,

but they weren't pushovers and
regularly revolted against the Inca.

So, maybe this
is an old Aymara site.

NARRATOR: The team eventually
uncovered over 20,000 artifacts,

but, about 40cm below ground,

something else grabbed
their attention.

HARIDY: It's human remains!

A body in a flexed position
oriented towards the east.

Buried with it
are two projectile points,

one is near the pelvic area and the
other is touching the right forearm.

- Based on the skull and jawbone,
it's determined that this is a male

and dental wear patterns
indicate that he was between

25 and 30-years-old when he died.

ELLIS: Given the style of the
projectile points that they found,

it's estimated that the remains are
between 7,000 and 9,000-years-old,

predating the Aymara
by thousands of years.

So, who were these people?

NARRATOR: Further excavation
of the site,

now named Wilamaya Patjxa,

revealed another four burial sites
containing human remains.

But, unlike the first grave,
they didn't contain any artifacts.

- This lack of grave goods
isn't really that surprising.

These people were likely
hunter-gatherers

who moved around a lot
and didn't carry much with them.

NARRATOR: As the team continued
digging,

they discovered another set
of human remains

buried around 85 centimetres
below ground.

ELLIS: It's fragments
of a skull, teeth

and parts of various leg bones.

But that's not all.

There are 24 stone artifacts
with the body,

most of them in a tidy pile
near the hip.

So, clearly this was a person
treated with reverence.

But who were they?

NARRATOR: About a kilometre
to the north of Wilamaya Patjxa,

another burial site had been
discovered 18 years earlier.

- It contained the remains
of 16 individuals,

two of whom showed evidence
of violent trauma.

One of the bodies had a large hole
in the left parietal bone

and a dart point was found between
the ribs and sternum.

LEONARD: It looks like this person
likely suffered a violent death.

Given all the projectile points
found in the area,

maybe there was a battle
between these two groups.

These highland communities would
have been exploring

new and fragile cooperative dynamics
that could have led to competition.

ELLIS: But radiocarbon dating shows
that the remains discovered

with evidence of injury nearby are
over a thousand years younger

than the ones found with the grave
goods at Wilamaya Patjxa.

NARRATOR: The team conducted
a study of the bones,

analysing their stable
oxygen and carbon isotopes.

The higher the elevation
where people live,

the lower the number of oxygen
isotopes will be present,

and the greater the number
of carbon isotopes.

- Based on the results,
it's determined that

these people lived permanently
at high altitude,

but likely lower than
the Titicaca Basin.

So, it's likely that this area was
used for seasonal habitation.

- But why would they come here?

With the dry conditions, limited
resources and low oxygen,

it's not exactly a welcoming
environment.

NARRATOR: Further excavations
uncovered something unexpected,

given the area's
inhospitable reputation.

HARIDY: There are fragments
of animal bones,

mostly large mammals
like deer and camels,

but there are 106
unidentified fragments.

LEONARD: This is quite surprising
given the scarcity of vegetation

and overall dryness of the region.

It doesn't seem
to be able to support

such a diverse range of wildlife.

NARRATOR: As the team took a closer
look at the artifacts left

in the burial site
at Wilamaya Patjxa,

the role of the individual
became clearer.

- These are tools
for hunting big game.

Given the presence
of so many animal bones,

this area was probably used
as a seasonal hunting ground.

- The projectile points would've
been for bringing down the animals,

probably with an atlatl,
a spear-throwing device

used by ancient hunters.

- The knives would've been used
for removing the animal's organs

and the scrapers and stones were for
processing the animal's hides.

NARDI: Because most tools were found
in a neat pile next to the body,

they were likely placed there
in a container of some kind,

probably a leather satchel
that disintegrated over the years.

This was a symbolic act of respect
shown to a skilled hunter.

NARRATOR: The team attempted
to further analyse the bones

to gather more information,

but they were too degraded to be
studied for certain attributes.

However, a new form of testing may
be able to unlock their secrets.

A recently developed technique
examined the proteins

in the enamel of the teeth,

specifically, a protein
called amelogenin.

And the tested sample
provided a surprising result.

NARRATOR: In Peru,
on the shores of Lake Titicaca,

archaeologists examining the remains
from the burial at Wilamaya Patjxa

had a stunning realisation.

- The remains belong to a woman
between 17 and 19-years-old.

This is a major discovery

because it's often assumed that in
early hunter-gatherer societies,

the males did all of the hunting.

NARRATOR: This discovery wasn't the
first time a prehistoric female,

thought to be a hunter,
had been found.

In 1989, roadworkers accidentally
unearthed human remains

in a gravel quarry near the town
of Buhl, in south-central Idaho.

- Analysis of the bones
revealed that

the individual was
a Paleoindian woman,

who was roughly between
17 and 21-years-old when she died,

about 10,000 years ago.

- Along with the body,
they found several grave goods,

including a tool made of bone and
a large-stemmed projectile point,

similar to what was discovered
at Wilamaya Patjxa.

- It's not just these hunting
discoveries that are causing people

to rethink traditional gender roles
in the ancient world.

There's also significant
evidence to suggest

that some prehistoric women were not
only hunters,

but celebrated warriors
defending their people as well.

NARRATOR: A team of researchers had
recently performed DNA analysis

on the remains of a 13-year-old
Scythian child warrior,

discovered in Siberia in 1988.

HARIDY: For decades, the scientific
community assumed that

it was a male because they found
a quiver with arrows,

a birch bow and an axe
with the body,

grave goods that are traditionally
associated with males.

- But the DNA revealed that the
remains actually belong to a female,

thought to have died
around 2,600-years ago.

NARRATOR: In order
to expand on the significance

of the findings at Wilamaya Patjxa,

researchers analysed burials
across the Americas,

studying 429 sites

dated between 8,000
and 14,000 years ago.

- They found that only 27 of them
had the tools for hunting big game.

Surprisingly, 10 of these sites

contained the remains of females.

NARRATOR: Based on this data,
the team concluded that

between 30 and 50 percent
of prehistoric big game hunters

might have been female.

But some aren't convinced.

- It's a very small sample size

to arrive at such
a sweeping conclusion.

ELLIS: It's also important to
remember that

the interpretation of grave goods
is not so cut and dry.

Certain practices may differ
across cultures and generations.

LEONARD: But given the sheer
volume of hunting artifacts

found with the body,
it's safe to assume that

the female remains found
at Wilamaya Patjxa

belonged to a hunter of some renown.

NARRATOR: While the team admit that
further research is required

to substantiate the conclusions
of the study, the discovery,

along with the growing body
of evidence across the field,

may just force us to reconsider
our preconceived notions

about the role of women
in prehistoric societies.

The Arabian Peninsula features not
only extensive areas

of arid landscapes of sand and rock,

but also huge lava fields or harrat.

One of these fields named,
Harrat Khaybar,

covers 14,000 square kilometres
of Saudi Arabia.

- The Harrat contains a wide range
of volcanic rock types

and extraordinary landforms.

And there are over 400 kilometres
of unexplored passages

and at least 40 Lava Tubes here.

- These tubes are natural tunnels
created by a lava flow.

At the end of an eruption
and after the lava starts flowing,

it drains and leaves these
tunnels, caves

and entire cave systems
beneath the ground.

NARRATOR: While exploring and
mapping this fascinating landscape,

speleologists, or cave scientists,
ventured deep underground

inside a cave known as Umm Jirsan

and uncovered something
truly terrifying.

- Deep within this enormous lava
cave system

is a massive cache of bones,

hundreds of them!

And beyond what is visible,
there are presumably

many, many more underneath the
layers of undisturbed sediment.

- Not only is this
a horrific boneyard,

but animal snarls echoing from the
depths of the cave,

believed to be made by wolves,

are enough to send you
running for the surface.

NARDI: The volcanic field was formed
by multiple eruptions

along a 100 kilometre vent system
over a period of 5 million years.

And the most recent
eruption on record

happened between
1,300 to 1,400 years ago.

NARRATOR: Archaeologists were
called in to investigate Umm Jirsan,

the longest lava tube in Arabia,
measuring 1,481m.

Its length is divided into
three separate passages

by two areas of structural collapse.

The average height of the cave's
passages are 8 to 12 metres

with a 45 metre width.

- Umm Jirsan travels underground
from east to west.

The western passage, where the huge
cache of bones was found,

has only one entrance
and is 341metres long.

HARIDY: While there are
a few other smaller deposits

and scattered bones
in other areas of the tunnel,

the extraordinary
quantity of bones found

at the end of the western passage
are likely the work of predators,

who were creatures of habit
and tend to bring their prey back

to the same location
again and again.

- Given the piles of bones and the
accompanying, bone-chilling growls,

Umm Jirsan earned
the nickname 'Wolf Den'.

So, maybe that's what it is?

The den of some very well-fed
wolves.

- (wolves howling)

HARIDY: The Arabian Wolf is
a desert-adapted subspecies,

native to the Arabian Peninsula

and is a protected species
in Saudi Arabia.

NARRATOR: Arabian Wolves are
one of the smallest subspecies

of wolf on the planet.

Their small stature likely helps
them adapt to life

in a hot, dry climate.

MIFFLIN: While wolves
do use cave as dens,

they tend to stick
to the mouth of the cave.

So, a wolf den nearly 350 metres
underground

would be very out of character.

- Radiocarbon dating
of a selection of the bones

proves that they range in age
from 6,839

to 439-years-old.

This is a wide expanse of time in
which many more varieties

of wildlife would have been
living in the region.

NARRATOR: The bones were
inspected for weathering,

chemical corrosion, abrasion,

and carnivore or rodent damage.

They ranged from
completely recognisable

with scraps of dehydrated tissue
still attached,

to highly decayed bone fragments
and splinters.

But two skull fragments
were of particular interest.

These two skull-cap fragments are
undoubtedly human!

Could they be here as victims or
were they the predator themselves?

MIFFLIN: However, there is
no significant evidence

of human presence in the 'Wolf den'.

The volume of bones in the western
passage, including the human skulls,

are not likely the result
of human activity.

NARRATOR: Each bone
and fragment was examined

under 10 to 20 times magnification
with different exposures

to thoroughly study the remains.

Then, the bones with areas
of unique surface damage or wear,

were examined
under 80 times magnification

and showed compelling results.

- Forty-two percent of the bones
show evidence

of having been eaten by carnivores.

This includes gnaw marks,
partially digested bones,

and rounding from licking.

So, humans are definitely
off the table.

- Some of the tooth impressions
are linked with rodents

and there are a number
of distinct quills found

throughout the length of Umm Jirsan.

Perhaps porcupines,
known as the 'bone collectors'

of the animal kingdom,
could be responsible.

NARRATOR: The cave dwellings
of an African Crested Porcupine

are littered with hundreds,
if not thousands, of bones

and though small,
porcupines have been known

to take on and kill large cats.

HARIDY: While porcupines do gnaw on
items like bone and wood

to shave down the perpetually
growing incisors,

they are actually herbivores.

So, it's unlikely that a porcupine,
or even a prickle of porcupines,

would have taken down
so much large prey

and dragged them underground
just to gnaw on the bones.

MIFFLIN: Carnivores capable
of this kind of damage

include large cats like lions,
cheetahs and leopards.

Many of which are
critically endangered

or extinct in this part
of the world.

NARRATOR: Asiatic or Persian Lions

lived across the Arabian Peninsula
until the 19th century,

but now only survive in India.

The large Asiatic Cheetah
is now only found in Iran.

And there are less than 200
Arabian Leopards left in the wild

in the mountains of Saudi Arabia.

CANTOR: These big cats could have
been on the prowl in this region

during the early Holocene period

and would have enjoyed
the variety and quantity

of the remains found here
as part of a healthy cat diet.

However, big cats don't tend
to live in such deep caves

and they prefer dining on
their prey right out in the open.

NARRATOR: Turning back
to Umm Jirsan,

archaeologists examined
the tunnel floor for further clues.

Among the sediment
and guano deposits

they found hundreds
of fossilized faecal specimens

known as coprolites.

NARDI: The majority of coprolites
collected here

have a white appearance
with a crumbly texture,

and globular shape,
with either a single pellet

or fused pellet appearance.

This faeces is consistent with one
carnivore in particular...

the Hyena.

CANTOR: While not the largest
carnivorous predator,

what they lack in size,
they make up for in design.

Their distinct and powerful necks
and elongated front legs

allow them to pull and move kill
over great distances.

HARIDY: Striped Hyenas are
cave dwelling predators

who do call Saudi Arabia home.

They're a smaller species than
their famed African cousins,

weighing only 50 kilograms.

And Hyena poop is famously white,

due to the high calcium content,

as a result of digesting
so much bone.

MIFFLIN: They have
a digestive system

that kills the bacteria
in scavenged meat.

And their powerful jaws and razor
sharp teeth

can reduce whole skeletons,
hooves and all,

to these piles of fragments
and undigested bone shards.

NARDI: Then we have the cave itself,

Hyenas are not as nomadic
as many other predators,

and have a distinct family structure
that binds them together

in groups called 'Cackles'.

HARIDY: As such, Hyenas actually
spend a lot of their time

hanging out in cavernous dens

complete with the bones
of all the creatures they prey on.

One den was even found
490 metres deep into a cave,

that's 150 metres deeper than
the den at Umm Jirsan.

NARRATOR: Experts confidently
believe that the cache of bones

at the bottom of the western
passage of Umm Jirsan

belong to a Striped Hyena Den.

This is where the Hyena Cackle

not only filled the cave with
the evidence of their feasts,

but likely the wailing and
'laughter' Hyenas are known for,

which could explain the terrifying
sounds the discovery team heard

echoing from depths of the cave.

Across the huge,
inhospitable expanse

of North Africa's Sahara desert

is Tunisia's Gulf of Hammamet.

- Tunisia's climate varies
considerably due to the influence

of the Mediterranean Sea
and the Sahara Desert.

- Given its strategic location on
the Mediterranean,

it has seen an incredibly
diverse range of people

trading with, settling in
and even colonizing the area

over thousands of years,
giving it a unique history.

NARRATOR: In the days following
a very heavy wind storm,

beach-goers noticed something odd
emerging from the sand

in the shallow waters of the gulf.

Strange shaped stones
just beneath the waves.

- Wind and water are always
chipping away at coastlines,

altering these landscapes over time,

but these rocks seem entirely
out of place on the seafloor.

Perhaps they're man-made?

ELLIS: Strange items have been known
to wash up on shores after storms.

Everything from beached whales
and trash,

to shipwrecks
and bottled messages.

So, while these rock formations
are unnatural,

they are not entirely unique,
given the circumstances.

LEONARD: As word of
their discovery spread,

teams of interested investigators
descended upon the gulf

in search of answers.

NARRATOR: Divers took to the water

and quickly discovered that
these aren't displaced rocks,

nor are they rubbish
shifted by the storm.

These are blocks,

100% man-made building blocks

with right angles
and hewn, flat surfaces.

- Actually, given the amount of
algae present,

these stones have clearly been
underwater for a very long time.

so, where did they come from?

NARRATOR: Expanding their underwater
search,

not only did they find more blocks,

but they discovered connecting
foundations and structures

poking up from the sand
and sea growth.

- A few loose stones
could be explained.

But intact building foundations?

That's a surprise!

Whatever it is we're looking at,
used to be above the water.

So, what happened here?

NARRATOR: A discovery, lost to
the sands of Nabeul, Tunisia

had perplexed archaeologists.

Limestone has been extensively
used as building blocks

in the Mediterranean basin since the
first constructions made by humans.

Expanding their search on land

the archaeologists hoped
to encounter ruins

that could help explain
their findings.

- the grassy area just beyond
the beach

is a massive foundation
made of similar blocks

to the ones found in the water.

It's palatial, and appears to be
built around a central water feature

and courtyard structure.

WAKEFIELD: Its design is reminiscent
of classic Roman villas,

dated to the middle
of the 4th century.

LEONARD: It's as elegant and well
designed

as those found 1,300 kilometres
away in Rome.

Were the Tunisians fans
of Roman design?

Who built these and why?

NARRATOR: Thousands of years ago,

the entirety of
the Mediterranean Sea

and every surrounding piece of land
that touched the water

was absorbed into
the booming Roman Empire.

- Over 2,000 years ago,
the northern coast of Africa

was prized for its fertile
agricultural lands

and it became known as
'the Breadbasket of the Empire'

due to its abundant supply of grain.

- Considering the number
and quality of historic Roman sites

found across Tunisia,

the area was clearly favoured
by the elites of the Empire.

ELLIS: Palatial villas for the
wealthy, functional aqueducts,

ornate bathhouses and communal
sites for entertainment

were built throughout the country
at great expense.

WAKEFIELD: Signs of
the Roman 'good life' exist

across the gulf region of Tunisia.

But there are no ruins of farms
in the immediate vicinity.

So, where did this wealth come from?

NARRATOR: On the grounds of the
villa known as the 'House of Nymphs'

there is an intricately decorated
ceramic basin,

complete with a mosaic inlay,

featuring the image
of the God Oceanus

surrounded by fish
that could offer some insight.

- The art work implies
a bounty of fish from the sea.

Perhaps the seaside community
was flourishing

due to a thriving fishing industry.

NARRATOR: Teams of experts
were called in to examine the site

and using drone imaging,

they were able to create a connected
grid of all the known structures

on land and beneath the water.

The submerged discoveries
appeared to match the layout

and engineered design
of the aboveground ruins.

- This combines
the known area on land

with another 20 hectares
of structures in the gulf

into one large,
connected site or city.

LEONARD: Few historical
records exist for the area,

but the references
we do have indicate

this could be the rumoured,
Roman metropolis of Neapolis,

founded by Julius Caesar
after his war against Pompeii.

NARRATOR: Neapolis was originally
established in the 5th Century BCE

as a Phoenician outpost.

The Phoenicians were
an ancient civilization

of merchants, traders,
and colonisers

originating in
the eastern Mediterranean,

primarily located in modern
Lebanon and coastal Syria.

But, the city was lost
to the Romans in 146 CE.

- It's likely due to its frequent
and often violent

changes in government that there are
so few records of its history.

But with little to go on, and nearly
half of the site underwater,

investigations must continue.

NARRATOR: On the periphery of the
underwater site,

divers located a large foundation

with giant embedded tanks...

..hundreds of them.

WAKEFIELD: The tanks are
often built in sets of three,

they're about 2 metres deep,

and they are lined
with ancient concrete.

They appear to be connected
through pipes

from the outer tanks
to the inner, larger basin.

LEONARD: These tanks
are very familiar.

They've been found
throughout the Roman Empire,

but rarely in these numbers.

These are Garum tanks.

NARRATOR: Garum tanks are vats
where ungutted fish are salted,

seasoned, left to liquefy, ferment
and then dry in the sun.

- The bacteria present
in the fish intestine

is the secret ingredient
of the fermentation process.

It's a very smelly,
very long process

that results in the
collection of a clear fish sauce

in the central basin which is packed
with protein and flavour.

ELLIS: Historians believe it was
served along with meats,

vegetables and fruits, and was even
thought to be an aphrodisiac.

NARRATOR: Continuing the exploration
of the beachside area,

experts came across
another large ruin,

but this wasn't a Roman villa.

It appeared to have
a more functional design.

CANTOR: Along the grounds of this
large property are more garum tanks.

These above-ground ruins
even contain the pungent residue

of fish bones and guts.

So, it's likely this was
a Roman garum factory.

LEONARD: Garum was the favourite
condiment of the Empire

and due it's extreme salt content,
it could last for years.

The underwater discovery
of hundreds of tanks here,

combined with the tanks on land,

point to this city being
a major production hub.

ELLIS: If this were a successful
garum factory town,

the producers would need
some way to package and sell

their liquid gold
throughout the Empire.

Amphora was just the ticket.

They were a specific type of
two-handled ceramic vessel

used to transport liquids,
including Garum.

- Amphoras that are believed to have
been made in Neapolis

feature a ring-like rim,
moulded outer face,

with a thin, elongated neck,

and wide, three-grooved handles.

And they have been found
throughout the Roman Empire.

LEONARD: What we now know, is that
Neapolis became the hub

of Garum production
for the Roman Empire.

But there is something fishier than
the smell of Garum going on here,

how did 20 hectares of the city
end up underwater,

lost to the world for millennia?

NARRATOR: The discovery of
ancient garum facilities in Neapolis

explains the affluence
of the region at the time.

But, the mystery of the structures
found in Tunisia's Gulf of Hammamet

remains unsolved.

The Mediterranean
and northern coast of Africa

sit on very active tectonic plates

leading to increased
earthquake activity.

- It's possible a quake could have
wreaked this kind of havoc,

but there are no records or
physical evidence here

of any earthquake capable of
dropping a city into the sea.

- While no earthquakes were
registered at that time

along the African coast,

nearly 2,000 kilometres away
in Alexandria,

records show that the city was
devastated by a massive tsunami.

NARRATOR: On the 21st of July, 365CE

the Roman historian
Ammianus Marcellinus wrote

"..the solidity of the whole Earth
was made to shake and shudder,

and the sea was driven away,
its waves were rolled back,

and it disappeared...",

before returning to destroy
the city in its wake.

- Over 80 % of Tsunamis are
caused by earthquakes.

By following displaced soil
and sediment samples

from the bottom of
the Mediterranean,

experts are able to track
Alexandria's tsunami to its source.

ELLIS: It's believed,
based on the current evidence,

that the wave originated
as a result of an earthquake

at the island of Crete,
654 kilometres away from Alexandria.

- It's estimated that the quake
would have had a rating

of up to 8.5 on the Richter scale
in order to have caused

the devastation registered along the
coasts of north Africa,

Spain and Greece.
That's a huge earthquake.

And yet, no major wave
came to Neapolis's shores.

- So, it wasn't an earthquake
or a tsunami

that sank half the city.

Whatever happened in Neapolis

seems to have taken a slower
and more insidious path.

NARRATOR: Experts now believe that
the Crete earthquake

and subsequent tsunami created
another, unexpected consequence...

..an underwater landslide off
the coast of ancient Neapolis.

- If proven true, it would have had
a huge destabilizing effect

on the coastline of Tunisia.

As the undercurrent swept
sands deeper into the sea,

the ground along the coast would
have become infiltrated with water.

- The theory is that Neapolis
likely suffered from liquefaction,

which ultimately left the city
sinking into the sea.

NARRATOR: Liquefaction occurs when
loosely packed, water-logged soil

at, or near ground level loses
its strength or structural integrity

in response to vibration
in the ground.

CANTOR: The loss of major elements
of the city and its garum production

would have been a huge blow
to the Roman Empire,

which itself was already
nearing the point of collapse.

WAKEFIELD: Interestingly,
waves were the key

to finding the lost city
of Neapolis,

as well as its source demise
so many years ago.

NARRATOR: Today the modern
town of Nabeul,

which happens to mean 'Neapolis',

stands watch over what remains
of the ancient Roman city,

both above and below the sea.

A testament to the unique and
connective history of the area.

Towering 300 metres
above the expanse

of southern Mongolia's Gobi Desert,

the sand dunes at Khongoryn Els
are a dramatic change

from the otherwise
flat surroundings.

- The Gobi stretches across a
variety of different ecoregions.

Sand dunes give way to
hard-packed, rock desert

which transitions to
salt marshes and lakes.

- But one thing that the entire
region has in common

is how little rain it receives,

only about 40 millimetres a year.

LEONARD: That's an incredibly
low amount of rainfall.

To put things into perspective,
Australian deserts

average around
250 millimetres a year.

NARRATOR: The dunes are some
of the biggest and most magnificent

in Mongolia,
drawing people from far and wide.

But it's not just the dunes
that draw gasps of wonder.

Researchers visiting the area
encountered an otherworldly,

deafening sound that
made their bodies vibrate.

- (low-pitched humming)

CANTOR: It's all encompassing.

The sound is this low pitched drone
that sounds as though

it's coming from everywhere
all at once.

It's a rich baritone,

almost as if someone
or something is singing.

It's almost unbelievable that it's
actually happening.

But where could the sound
be coming from?

- It's a pretty frightening sound.

It adds to the mystery
of this entire area,

which occupies a dramatic and
important space in human history.

MIFFLIN: Over the course
of centuries

travellers and tradesmen alike
struggled their way across

this ruthless environment.
And because of this,

we have many accounts detailing
their experiences.

NARRATOR: The researchers began to
investigate this auditory phenomenon

that has mystified everyone
who has ever encountered it

since it was first recorded in a
9th century Chinese manuscript.

- In the 13th century, Marco Polo
also famously wrote about

an inexplicable sound that terrified
everyone who encountered it.

CANTOR: The 'singing' can last
for more than a minute.

And it registers at a
pretty low frequency

and at over 100 decibels.

This is incredible,
it's louder than a jet engine!

MIFFLIN: The sounds appear to be
originating from far away.

Some people say that they're
actually spirits

residing between the dunes,

looking to lure people away
from their companions.

WAKEFIELD: It would be hard to prove
that spirits are at work here.

And although there are plenty of
things in this world

that can't be explained,
scientists believe

that this is not one of them.

MIFFLIN: If you've ever heard a
swarm of locusts or cicadas

you might think that they could
cause such a disruptive sound,

but when you listen closely,

these sounds in the desert
are very different.

- And while earthquakes have been
known to make loud, booming sounds,

there's been no seismic activity
picked up around the dunes

when this sound occurs.

So, if it isn't a result of animals
or plate tectonics,

you wonder if it could be sand
that has something to do with it.

It's the one thing there's
a lot of around here.

NARRATOR: This bizarre
audio phenomenon

has been puzzling the scientific
community for many years.

Their search for answers led them
to the continent of Australia.

The sands at Hyam's
and Whitehaven Beach

famously emit loud squeaks
when people walk across them.

- (sand squeaking)

- But these mousy squeaks are much
shorter than the meditative sounds

heard among the dunes of Mongolia.

They're also more high-pitched.

MIFFLIN: They don't really
compare whatsoever.

Measurements reveal that they are
in the 500 to 2,500 hertz range,

whereas these loud sounds in
Mongolia measure below 300 hertz,

a much lower frequency.

- Of course, this is a totally
different sound,

but could the physical mechanisms
causing the sand to squeak

be similar to what's happening
in the Gobi Desert?

NARRATOR: Examination of the sands
at the Australian beaches

revealed that this high-pitched
sound only happens

if the sand grains are composed of
quartz and have a rounded shape.

CANTOR: The squeak happens when
the similarly-sized quartz grains

rub against each other
and start vibrating.

The vibration then transfers
its energy into the tiny air pockets

surrounding the sand grains.

This energy then reaches our ears
in the form of a squeak.

MIFFLIN: Also, the sand only squeaks
as a result of someone

or something interacting with it.

But in the Gobi, even if the
scientists don't make a move,

this noise will still occur.

NARRATOR: Research reveals that it
is not only in Mongolia

where people have
witnessed the phenomenon.

The singing sands
have been encountered

in many of the world's deserts.

- There are reports of these
booming or singing sounds

in the deserts of Morocco and Oman.

And in the US, the Eureka
dunes of Death Valley

have mystified visitors for years.

NARRATOR: The Eureka sand dunes
are the tallest dunes

in the state of California.

They rise approximately
200 metres from the desert floor.

Researchers noticed that
when the wind picked up,

these strange sounds
increased in frequency.

- Wind can cause a whole range
of sounds,

but it doesn't have the capacity
to cause these feelings

of vibrations inside the body.

WAKEFIELD: While observing the dunes
they notice that sand avalanches

are triggered by the wind.

A sand avalanche happens
when the angle of the sand

accumulated at the top
of a dune reaches a critical point

where it's incapable
of supporting its own weight.

- Usually a couple of seconds
after the avalanche,

this booming, singing sound occurs.

So it is possible that
the sand's movement

is somehow causing these sounds.

NARRATOR: In order to better
understand

what processes were taking place,

the scientists used an instrument
known as a geophone,

a device used to measure vibrations.

If the Earth, or in this case
the sand is vibrating,

the geophone will detect this.

They placed the geophone,
not on the face of the dune,

but on the flat ground beneath it.

They then triggered
a sand avalanche

and waited to see if the geophone
registered anything.

- Sure enough, the geophone
registers the vibrations rippling

through the dune and continuing
along the valley floor.

So the sand is somehow
causing these sounds,

but there has to be more
at play here.

LEONARD: Additionally, when the sand
of the valley floor is agitated

the geophone picks up
the vibrations,

this means that the sand grains are
rubbing against each other,

but most importantly, the tell-tale
drone remains absent.

So, whatever is happening
must exclusively involve

the structure of the sand dune.

NARRATOR: Hoping the dune itself may
shed some light on the mystery,

the researchers considered
its structure and formation.

MIFFLIN: A dune is essentially
created by wind blowing sand

across a landscape. As it does,

little piles begin to accumulate,
and as more and more sand gathers

on top of these piles,
a gap is formed in front of them.

- As it grows, it does so in layers.

And we're talking tons of sand here.

Remember, these dunes
are hundreds of metres tall.

As the dune gets bigger,
the sand in its core is compacted

by all of the sand that's above it.

NARRATOR: A dune can
only reach a certain height,

when an opposing side reaches
an angle of around 33 degrees,

it will 'self-regulate',

releasing avalanches
that cascade down its face.

LEONARD: Ground penetrating
radar reveals that

the top layer of the dune is around
one and a half to two metres

of loosely packed sand.

Below that, is a super densely
packed mass of sand

that's almost like concrete.

But, it's on top of the outer layer
where the avalanching occurs.

NARRATOR: Curious about the
properties of this top layer,

the scientists looked
at its seismic velocity,

its ability to carry seismic waves.

- The physical properties of
the dune's top layer

allows the vibration to travel
through it at a specific velocity,

while the dense,
concrete-like sand below it

only reflects these vibrations
right back at it.

NARRATOR: The scientists theorise
that as the sand is avalanching,

the vibrations created by
the moving sand are trapped

and reverberate throughout
the top layer.

CANTOR: The top layer is acting
like an amplifier of the vibrations

caused by the sliding sand.

This has the same effect that the
body of a guitar has.

When you strum a chord, you don't
get any reverberating sound

unless you have the body of
the instrument to capture it.

- Amazingly, it creates
its own feedback loop,

which, as we have heard, can be both
terrifying and beautiful.

NARRATOR: The phenomenon
of the singing sands

can be characterized
as nature's true instrument.

Playing these long,
drawn-out notes for humanity

since time began.

Subtitles by Sky Access Services