Original subtitles

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