Original subtitles

NARRATOR: Drone footage reveals remnants from the past.

- What is this place? And why's it underwater?

NARRATOR: An aerial view uncovers a puzzling pattern

and an eerie mystery.

- Who was John Greer?

And why is his tombstone in the middle of the Gulf of Mexico?

NARRATOR: Eyes above the ocean floor capture an incredible sight.

- If it is a road how old is it?

NARRATOR: Satellite images shed light on an ancient mystery.

DAN: The belief is that these formations could be as old

as 40,000 years!

NARRATOR: Everywhere we look on our planet,

there's evidence of the past.

In nature.

In buildings.

In relics.

Each holds a mystery that technology now allows us to see from above.

What new secrets are revealed?

Water. It covers more than 70% of the Earth's surface.

And yet we know more about the far reaches of space

than we do about the depths of our own oceans.

But the view from above is beginning to lift the veil

on these hidden places and revealing new mysteries.

Just off the coast of Italy, in the Gulf of Naples,

a view above the blue waters reveals something incredible.

Could a closer look yield clues?

Just below the water's surface a clearer picture begins to emerge.

CYLITA: Divers have found rich marble floors

and mosaic tiled pools

hidden for centuries under the sand.

GUY: Now, what's incredible is that this site covers

an area of more than 170 hectares.

NARRATOR: A massive expanse of submerged ruins

and something even more haunting,

scores of sunken statues.

- What is this place? And why's it underwater?

NARRATOR: Can the view from above provide answers?

Found along the neighbouring shoreline are more ruins.

These are remnants of an ancient Roman city known as Baiae.

DAN: But somewhere along the line, half of this harbour city

got swallowed by the sea and lost for centuries.

NARRATOR: What led to this unfortunate fate?

And what else can be learned with a view from above?

GUY: Now if you want to begin to unravel the mystery of Baiae,

understanding its location is key.

Baiae is located on the coast of Italy,

south of Rome, which, of course,

was one of the greatest capitals of the ancient world.

SHEILA: It was from this place that rulers,

politicians and businesspeople

controlled the Roman Republic and later the Roman Empire -

one of history's largest powers.

NATASHA: And Baiae is where these important players

would come in search of something the Romans called "Otium."

SHEILA: "Otium" can be translated to "a pause", "a break", or "leisure".

GUY: To put it simply,

this is where Rome's rich and powerful went on holiday.

NARRATOR: And it's where many of Rome's elite

built their vacation homes.

- You might recognise the names of some of the property owners

including Julius Caesar,

politician Mark Antony, the statesman and Philosopher Cicero,

as well as the infamous Roman Emperor Nero.

GUY: Given the enormous amount

of wealth and power concentrated here,

this seaside resort evolved or, perhaps we should say,

devolved into a kind of Sin City.

NATASHA: The writer and philosopher Seneca the Younger

once described Baiae as a "vortex of luxury"

and "harbour of vice."

- The story of Baiae does have parallels to the legend of Atlantis.

NARRATOR: The Greek Philosopher Plato

famously wrote about a great civilisation

that sank beneath the sea.

And while his tale of Atlantis is understood to be a myth,

the similarities are uncanny.

SHEILA: Plato's moralistic tale talks about this advanced society

that becomes morally bankrupt with its people

devolving into immoral pursuits.

- And as Plato explains,

this behaviour angers the gods, it does not go down well with them.

And so they send down punishment

in a "terrible night of fire and earthquakes".

And a punishment that caused Atlantis to sink forever

beneath the waves.

NARRATOR: What was it that seems to have caused Baiae

to suffer a similar fate?

Once again, could the view from above open a window to the past?

This is known as the Imperial Villa

and over the years was used by several Roman emperors.

GUY: Perhaps it's because of its place - high on a hill -

that this Imperial Villa is one of the few that seems to have avoided

Baiae's unfortunate fate.

GEORGE: This place was built way back in the 1st Century BCE.

And if you think home theatres are impressive today,

this place included a Greek-style theatre

that could host 1,400 or so of your closest friends.

- The estate would have also included magnificent water features

and marble statues,

not unlike the statues found within the bay below.

NARRATOR: As researchers continue to discover the scope and scale

of these impressive villas and seaside mansions,

one question remains: Why are they all under water?

What more can the view from above teach us?

NATASHA: The area was renowned for its natural beauty

and panoramic views of the coast,

but there was another important draw.

NARRATOR: On the shores, high over the sunken ruins -

and in a part of Baiae that was spared disaster -

the view from above reveals an important piece to this puzzle.

SHEILA: This massive concrete dome is known as the Temple of Mercury.

It measures over 70 feet in diameter.

And when it was built it was the largest dome of its kind,

pre-dating the Pantheon in Rome.

GUY: For its time, this design would have been

seriously state of the art.

SHEILA: But, while the Dome's cutting-edge architecture

speaks to Baiae's importance,

for centuries its function was misunderstood.

NATASHA: Early archaeologists first identified it as a temple

dedicated to Mercury - the Roman god of finance and commerce.

- And nearby they found two other domes

that they called the Temple of Diana and the Temple of Venus.

- Now, for years it was believed that these domes

must have been important Roman temples,

but that was wrong.

Eventually, it was discovered that these buildings

were actually enclosures for pools.

DAN: These weren't temples at all, they were Roman baths!

And they were part of a much larger spa complex.

SHEILA: But why build it all here?

Because they were able to exploit a particular local resource.

NATASHA: That's because this entire area

is rich with geothermal hot springs.

The engineers that built these spas managed to harness

the region's geology to provide the hot water and steam they needed

for the spas' luxurious baths and saunas.

GEORGE: But often when you've got places like this,

full of hydrothermal activity, there's a catch.

And that's volcanic activity.

DAN: And that is definitely the case here.

In fact, this is one of the most seismically active

places on the planet.

NARRATOR: In fact, Baiae is surrounded by several volcanoes.

And one of them is notoriously destructive.

About 30km from the ancient spa town

sleeps the powerful and deadly volcano of Mount Vesuvius.

GUY: In the year 79 -

when the people of Baiae were likely to be partying hard -

you have Mount Vesuvius erupting with little to no warning.

- And this violent volcanic eruption buried several nearby cities

including Herculaneum, Oplontis, Stabiae, and Pompeii.

Pompeii, of course, is the most famous of these, in part,

because of those ghostly casts

that preserve the forms of its many victims.

NARRATOR: Could these haunting scenes

somehow relate to the underwater spectacle found in Baiae?

An area of ruins three times the size of those at Pompeii?

GEORGE: Despite its proximity to Mount Vesuvius,

Baiae was luckily spared from its destruction.

But there was another danger lurking from beneath.

NARRATOR: A danger hiding in plain sight

now visible with a view from above.

CYLITA: The town was built on top of

something called the Phlegraean Fields,

which, at first glance, doesn't look like much.

DAN: But for kilometres underground and under the water,

this "field" is festering with steam and pools of boiling mud.

CYLITA: And the reason this happens is that below the surface

this is one of the world's 20 or so known super volcanoes.

GUY: If, and frankly when, this volcano erupts,

it would likely be of a volume thousands of times more powerful

than a regular volcano.

DAN: We're potentially talking about an explosion

of hundreds of cubic kilometres of material.

CYLITA: It would be so big that it would make the eruption

that destroyed Pompeii look miniscule.

GEORGE: With a conical-shaped volcano, like Mount Vesuvius,

the eruption is pretty much always going to spout from the peak.

But with these Phlegraean Fields, the eruptions are random -

they can come from pretty much anywhere.

SHEILA: Thankfully, this super volcano

hasn't had a major eruption in centuries.

And so isn't the reason parts of Baiae are underwater.

NARRATOR: If a massive eruption didn't lead to the sinking

of this Roman vacation spot, what did?

NATASHA: A disaster did strike Baiae

and it has to do with seismic activity,

but unlike the destruction at Pompeii,

it happened relatively slowly, over hundreds of years.

CYLITA: It's a phenomenon known as bradyseism.

And it's what's caused half of this beautiful city

to find itself underwater.

DAN: Sometime in the 3rd or 4th century,

underground magma chambers started to empty.

And over time, that made the ground above them sink.

NARRATOR: But incredibly this sinking process seems to be...

reversing.

DAN: When the magma chambers empty,

the Earth's crust slowly falls, that's called negative bradyseism.

And when they fill up, causing the Earth to slowly rise,

that's positive bradyseism.

CYLITA: And it appears that that's what's happening now, albeit slowly.

The gradual refilling of these chambers

has so far brought the submerged ruins

back towards the surface, by something like six feet!

GUY: Now, it may not happen in our lifetime,

but one day, that once great city of Baiae

might just return to the surface.

NARRATOR: In the Gulf of Mexico, nearly 200km

off the coast of Florida, is a small group of sandy islands.

It's an area steeped in history and shrouded in mystery.

This is Dry Tortugas National Park.

KAREN: These keys were discovered in the early 1600th Century

by Spanish explorer Ponce De Leon.

CYLITA: After finding the islands brimming with wildlife,

he named them Las Islas de Tortugas

or "The Islands of Turtles".

TORRI: The location is home to historical Fort Jefferson,

and has been a significant site throughout American history.

NARRATOR: And it's here, from high above Fort Jefferson

and the small cluster of sand keys

that a new mystery unfolds.

In 2016, while flying over Dry Tortugas,

a park employee catches a glimpse of something strange

just below the surface of the sea.

- I was looking at the water,

and so while we were passing over, close to the fort,

I happened to spot a pattern in the water.

And then I see a series of dots.

And the dots were very geometrically shaped, almost like a square.

As an archaeologist,

I've been taught to pay attention to patterns in nature.

Oftentimes, it's worth another look.

CYLITA: What is this square pattern in the water?

It definitely doesn't appear to be a natural formation.

TORRI: How long has it been sitting here?

And how many people have flown over this same spot and missed it?

GUY: Now, it's not everyday that you look out from an aeroplane

and you discover an entire mystery!

NARRATOR: But it wouldn't be the first time that secrets

have been revealed in the depths of these waters.

GUY: Dry Tortugas sits

in the middle of an area rich in maritime history,

and it's a known hotspot for historic shipwrecks

and even lost treasure.

KAREN: Ever since the Spanish landed here in the 1500s,

the area has been a major shipping channel.

- But Dry Tortugas sits in the middle of fairly shallow water,

full of coral and rocks.

So, hundreds of years ago when you factor in stormy weather,

strong currents, and a lack of reliable maps

and navigational systems, this often proved to be disastrous.

- To put it simply, this place is a ship trap.

KAREN: Dry Tortugas, and the surrounding area,

is littered with sunken vessels.

Since the 16th century,

there have been more than 250 documented shipwrecks.

NARRATOR: Including one of the worst maritime catastrophes in history.

TORRI: In the 1620s, a fleet of Spanish ships -

loaded with treasure -

left from Havana, Cuba, heading back to Spain,

when they were hit by a powerful hurricane.

CYLITA: Seven ships were sunk in the storm,

and hundreds of people were lost,

along with the massive cargo containing

gold, silver and some of the finest emeralds in the world.

KAREN: The Spanish were able to retrieve some of the lost treasure.

But one vessel, the Nuestra Senora de Atocha,

was lost for over 300 years.

GUY: But in the 1980s, it was discovered by a treasure hunter

named Mel Fisher.

Now, he managed to find remains of the shipwreck,

along with what's been called "the Atocha Motherlode".

Now, that is a cache valued at a whopping

450 million dollars.

CYLITA: Even still, it's estimated that Fisher only found

about half of the sunken treasure.

So, much of it may still be out there.

NARRATOR: Could this strange pattern - seen from above -

be the sign of a lost shipwreck?

Even underwater treasure?

Or could it be something else altogether?

This is exactly what Josh Marano was determined to discover

when he set out with a dive team to get a closer look.

- The first thing that we actually found was

a single post in the ground.

And then we found another one, and another one.

NARRATOR: The mysterious underwater posts were clearly

what created the square pattern seen from above.

JOSH: I honestly expected it to be the remains of a lighthouse -

some kind of tower that was constructed on the water

to warn mariners away from shallow areas.

NARRATOR: But during their survey, the team discovers something else.

Covered in weeds and sediment, a large stone slab.

- The stone was greywacke, a type of dark sandstone.

GUY: Now, greywacke is a material that was used in the construction

of Fort Jefferson, primarily as flooring.

So, I'm wondering could this somehow be related to that?

NARRATOR: Far from lost treasure are the submerged posts and stone

nothing more than scrap material from the fort's construction?

But a closer look revealed something surprising.

The stone had something inscribed onto its surface, a name -

John Greer.

JOSH: And it actually had a date of death - November 5th 1861.

NARRATOR: What the team had found was a gravestone.

- Initially discovering the gravestone, I think,

everybody was just in a state of shock.

We really did not expect to find anything like that.

NARRATOR: What began as a discovery from above,

quickly lead to an eerie mystery down below.

- Who was John Greer?

- And why is his tombstone in the middle of the Gulf of Mexico?

NARRATOR: Is it possible that nearby Fort Jefferson holds a key?

GUY: Fort Jefferson is a massive coastal fortress.

And it's built right on top of one of the sand keys at Dry Tortugas.

CYLITA: The fort covers approximately 10 acres

and is made up of more than 16 million bricks!

- It was built to last, and for good reason.

TORRI: Florida was primarily ruled by Spain

from the 16th to 19th centuries.

And it was only acquired by the United States in 1819.

KAREN: Planning for fortification began almost immediately

after the Americans obtained control of the area.

CYLITA: This location was considered an important strategic point

for controlling the Straits of Florida and the Gulf of Mexico.

GUY: And the primary objective, at least initially,

was to ward off pirates.

You have, for hundreds of years,

pirates and mercenary ships roaming the area,

attacking merchant vessels all along the coast of Florida.

CYLITA: So, in the 1840s,

the Americans got to work on building Fort Jefferson.

GUY: But in the early 1860s,

we see the start of the American Civil War.

And, of course, the Union in the north does not want the fort

to fall into the hands of the rebels in the south.

KAREN: So, they sent men down to not only protect Fort Jefferson,

but also to work to bolster its security and improve its defences.

TORRI: Now, over the course of the Civil War,

Fort Jefferson was also used as a military prison.

In fact, by the end of the war, there were more prisoners

living there than soldiers.

KAREN: And the surrounding islands were used to accommodate

military personnel, workers, their families

and enslaved people, among others.

- So, these small sandy keys were becoming increasingly populated.

NARRATOR: Was John Greer among those who made up the growing population

at Dry Tortugas?

- We had a name and we had something to go to

and actually go back into the archives and research more.

And so John Greer was labelled as a quote, "white labourer"

on payroll records at the time.

And he was employed primarily as a carpenter,

basically working to build the fort.

CYLITA: But how did John Greer die?

And why is his gravestone underwater?

NARRATOR: Is it possible that his death had something to do

with a known killer on the islands?

GUY: You have outbreaks of Yellow Fever

hitting the United States several times during

the 18th and 19th centuries.

CYLITA: Yellow Fever is a mosquito-borne disease

that was brought over from the Caribbean via ships.

TORRI: And it caused absolute panic in the United States,

not to mention 100 to 150 thousand deaths.

KAREN: As the number of people at Fort Jefferson grew,

so did the risk of spreading and contracting this deadly disease.

GUY: Outbreaks at Fort Jefferson killed scores of people

throughout the 1860s and 1870s.

TORRI: To deal with this problem, some of the other islands

in Dry Tortugas were used to build makeshift quarantine hospitals.

CYLITA: This was good thinking.

Isolating the sick on other islands

likely saved hundreds of others from dying.

NARRATOR: Is it possible that John Greer was an unfortunate casualty

of this deadly outbreak?

- We did suspect that maybe he's a yellow fever victim,

but we have not been able to find any record of him in the hospital.

The very last record of him at Dry Tortugas

was actually a record where he did not pick up his last paycheque.

And that was November 1st of 1861.

Maybe he died in a more violent manner, maybe a work site accident?

But until we do additional research,

we may not know exactly how Mr Greer passed.

NARRATOR: While the circumstances surrounding his death are murky,

something else remains unclear,

why his tombstone was found underwater.

What other clues might be found with a view from above?

CYLITA: When you take a look at Dry Tortugas today,

it's made up of 6 or so islands.

But this wasn't always the case.

KAREN: When Ponce de Leon first arrived here in the 1500s,

there were actually eleven islands!

JOSH: These are very dynamic, very mobile sand islands

that move almost constantly.

In fact, the only reason that some of the islands are still in place

is because of the structures that were built on top of them.

CYLITA: The sand keys of Dry Tortugas

are constantly changing in size and shape,

and the number of distinct land masses continually varies.

TORRI: These changes are caused - in part -

by the effects of climate change,

as well as major storm events and hurricanes.

KAREN: And, over time, some of these sand keys

have eroded to a point

where they've disappeared beneath the waves.

NARRATOR: Could this submerged archaeological site,

with its strange pattern of posts and mysterious tombstone,

be the location of a former island?

JOSH: I immediately went back to historic charts of the area

and was able to figure out pretty quickly that there was a structure

on what was formally an island, but that structure

was not a lighthouse, but it was actually a hospital.

And that island has not been above water

for about the last 100 years.

They had actually described building a quarantine hospital

in that location with very specific measurements.

It was 30 by 34-foot stilt structure

that was, quote, "hurricane proof".

NARRATOR: With this historical information in hand,

the research team could reference satellite imagery from Google Earth

and measure the pattern of posts,

and sure enough, they measured 9 by 10 meters.

The same dimension as the former hospital building.

JOSH: So this island was one of the ones that they utilised

to try to pull sick individuals away from the fortification

and prevent them from infecting one of the 2,000 other individuals,

either garrisoned or imprisoned within the fort.

NARRATOR: And the archives turned up other documentation from the keys

including photographs from the island.

Showing that at one time this was one of Fort Jefferson's Cemeteries

and home to dozens of graves.

- And now that we have been able to do some more background research,

we've been able to determine that there's more than 70 people

in the water at that location.

And so, for me personally, it scores pretty high in my career.

I don't think I'll be able to beat this easily.

TORRI: It's a bit eerie to think about this 19th century cemetery

hidden underwater.

That dozens more gravesites are likely resting here,

waiting to be discovered.

CYLITA: It amazes me that had the pattern of posts

not been seen from above,

the memory of this former island hospital

and the people interred here

may have been completely lost to time.

KAREN: With its history of shipwrecks and naval disasters,

this area sadly made a watery grave for hundreds of people.

And ironically, even those who were once buried here on dry land

have, ultimately, had their final resting places

reclaimed by the surrounding sea.

NARRATOR: In a remote pocket of the North Pacific Ocean,

not far from the Hawaiian islands

and more than a kilometre below the surface...

..spotlights from a remotely operated vehicle

Illuminate something truly bizarre.

- What on Earth is that?

- Is that a yellow brick road?

- Are Dorothy and Toto about to show up at the bottom of the ocean?

NARRATOR: Aboard a mothership at the surface,

scientists watching a live feed are just as astounded.

SCIENTISTS: (ON RADIO) Woah! - Wow, look at that!

- What is that?

- The yellow brick road? - This is the yellow brick road.

NARRATOR: This peculiar pathway was discovered in 2022

in the Papahanaumokuakea Marine National Monument.

- Say that three times fast!

- What it is, is one of the largest

protected conservation areas in the world.

DAN: It covers roughly 1.5 million square kilometres

of the bottom of the Pacific ocean.

That is about the same size as the state of Alaska - it's big.

GEORGE: It's home to the most beautiful and bizarre creatures -

like gulper eels and anglerfish.

- There are stunning corals, and strange rock formations.

NARRATOR: And this mysterious yellow brick road.

NATASHA: It was discovered by researchers

with the Nautilus Exploration Program,

which is this amazing initiative to survey,

map and study the ocean.

DAN: For more than 10 years

scientists have been exploring different areas,

and my favourite part of all this is that most of the time

when they send remotely operated vehicle down there on a mission,

they livestream it so anyone in the world can get in on the adventure.

SHEILA: We know so little about the ocean,

despite the fact that it really is the lifeblood of our planet.

DAN: Some of the things they've found are beautiful,

and some of them are downright strange.

Here's a perfect example of strange - a yellow brick road!

NATASHA: The road is located at the top of a seamount

which is a fancy word for an underwater mountain.

TORRI: Now, when video of it was posted online

it caused quite a frenzy -

lots of comments about it being the lost road to Atlantis,

a path to sunken treasure and extreme riches.

DAN: I can tell you this - no treasure was found nearby,

but maybe it's an archaeological treasure.

NARRATOR: Could this 'yellow brick road' be evidence

of human engineering, hidden beneath the ocean for years?

Perhaps a clue can be found in the Caribbean Sea

just off the coast of southeastern Jamaica

where divers capture underwater footage

of a sight that looks eerily familiar.

- This also looks a lot like our Yellow Brick road.

- If it is a road, how old is it?

SHEILA: The site lies just offshore

near a small Jamaican fishing village called Port Royal.

GEORGE: While Port Royal is a sleepy fishing town now,

it was once a bustling port and trading post.

And a very important one.

NATASHA: In the 1600s, Port Royal was a major economic hub,

controlled by the English.

TORRI: But one day, in 1692,

this booming trade hub came crashing down. Literally.

- A powerful earthquake sent three massive shocks through the town

followed by an enormous tsunami.

NATASHA: It only took a few minutes,

but when it was over,

most of the town had been consumed by the sea.

SHEILA: It's estimated that the event killed 5,000 people.

2,000 perished instantly

and the rest died soon after, from related injuries and illness.

GEORGE: Over 30-acres of the original 51-acre town

sank under the waves, some parts as deep as 35 feet.

And they're still here, under the water.

NARRATOR: The remains of a once-bustling trade port,

hidden below the surface for over three centuries.

Could a similar disaster have led to this underwater mystery?

TORRI: This Yellow Brick Road differs from the underwater ruins

at Port Royal in a couple of significant ways.

First, it's more than 80 times deeper

and it's smack dab in the middle of the ocean.

This isn't a road that ever existed on land -

it's got to be something else.

NARRATOR: If this yellow brick road wasn't part

of an earlier human settlement,

is it possibly a natural formation?

- And if so how was it made?

SHEILA: Is Mother Nature capable of creating geometric shapes

like these "bricks"?

Absolutely. When you look closely, you start to notice it everywhere.

GEORGE: Sometimes these geometric formations are incredibly small,

and other times you find patterns that are unbelievably large.

NARRATOR: Like on the island of Tasmania,

where the view from above reveals rectangular formations

that look strikingly similar to the yellow brick road.

NATASHA: This patterned slab of rock is located in Pirate's Bay

on the Tasman Peninsula.

SHEILA: In the 1800s,

the Tasman Peninsula was best known as a penal colony -

housing hardened criminals - now it is a nature lover's paradise.

- One of the big draws for outdoor enthusiasts and photographers

is the tessellated pavement in Pirate's Cove.

NARRATOR: This 'tessellated pavement'

began to take shape millions of years ago

when a flat slab of siltstone

was fractured due to movements in the Earth.

As salty sea water poured over the rock,

it seeped into the cracks.

When the tide went out, the water evaporated

and salt crystals formed.

Those growing crystals exerted pressure on the rock

causing it to flake away,

leading to the exaggerated brick-like appearance we see today.

DAN: This amazing natural feature is the result of time, pressure,

salt and erosion.

So could those same elements be at play with the yellow brick road?

TORRI: Given that it's in the middle of the ocean

and over a thousand meters below the surface,

it's more than likely a natural formation.

NARRATOR: Perhaps a view above

the nearby Hawaiian islands will offer answers.

Specifically, over the southeastern shore of Hawai'i Island,

home to Kilauea.

GEORGE: Kilauea is one of the world's most active volcanoes.

It's been rumbling and erupting almost constantly for 40 years.

TORRI: Its last really big eruption event was in 2018.

Lava was spewing 50 meters in the air,

roads were swallowed and more than 700 homes were destroyed.

NARRATOR: Lava is a formidable force.

But there is one thing powerful enough to stop it

in its fiery tracks, water.

GEORGE: When lava and water collide, there are often explosions

and giant clouds of steam.

DAN: The lava can harden into these incredible formations,

and the seamount where the yellow brick road is found

is proof of that.

GEORGE: Seamounts are formed when magma rises through cracks

in the Earth's crust and erupts at the sea floor.

Eventually, the volcanic activity subsides,

and what's left is a mountain of hardened lava.

The seamount where the yellow brick road is located

is millions of years old,

but if you really want to appreciate how it formed,

you can pop down to the South Pacific where magma

is rising through the Earth's crust right now

at an active undersea volcano.

TORRI: The Kavachi Volcano clearly demonstrates

that when lava and water meet,

explosive things happen.

DAN: This is one of the most active undersea volcanoes in the world.

The first recorded eruption was in 1939,

and there have been nearly 40 eruptions since.

GEORGE: Sometimes you can see volcanic steam and ash

blast through the surface of the water,

other times scientists know it's erupting

by looking at satellite images that show discoloration in the water.

NARRATOR: The collision of lava and water can create a whole host

of incredible formations...

large seamounts and underwater volcanoes,

to more delicately shaped towers, caves and vents.

GEORGE: But the bottom line is that mother nature can create

magnificent sculptures when she combines water and lava.

NARRATOR: Even a yellow brick road?

GEORGE: We know this is natural,

but I've never seen underwater rocks this colour and shape before.

DAN: This combination of colours and shapes is unique,

but if you look at those elements separately,

you can see that other structures in the area have similar features.

SHEILA: This cliff face at a neighbouring seamount

has similar colouration.

And this basalt rock has a well defined brick-like pattern.

The yellow brick road was only found recently.

As far as scientific discoveries go, it's a baby.

PETER: Some of our planet's most unique features

are studied for years

before scientists can say exactly what they are,

or how they came to be.

TORRI: No one is ready to say with 100% confidence

what this yellow brick road is,

but based on known science, there are some probable explanations.

DAN: At some point, millions of years ago,

scorching lava likely came into contact with water

and then rapidly cooled in a process called quenching.

TORRI: As it cooled and solidified,

fractures would appear giving it its brick-like appearance.

It's sort of like when mud dries, hardens and cracks.

GEORGE: Over time, the surface of the hardened lava

might have experienced a chemical reaction,

similar to when rust forms on metal,

that resulted in its distinctive yellow hue.

This yellow brick road is another one

of Mother Nature's incredible masterpieces.

TORRI: As humans continue to study the seafloor from above,

who knows what we'll find.

DAN: The ocean covers about 70% of the planet's surface,

and we've only explored about 5% of it.

Honestly, that's one of the most

exciting things about the world today.

Everybody's obsessed with Mars,

but we haven't even finished exploring the Earth yet.

It's amazing.

NARRATOR: The view above Australia

reveals something curious off the coast.

Scattered throughout the blue waters of the Gulf of Carpentaria,

a series of unusual structures.

- They appear to be some sort of rock formation.

DAN: It also looks like they're partially submerged in the water.

Some of these things have been measured at a metre high

and over 280 meters long.

NARRATOR: In all, over 300 of these formations

have been discovered along Australia's shores.

And as the view from above reveals,

they don't seem to be limited to the country's coastlines.

600 kilometres inland from Sydney,

in New South Wales, satellite imagery reveals similar shapes,

this time, found within a river known as the Barwon.

DSN: At this site, the shapes seem to stretch and snake

along the riverbed for about a half kilometre.

NARRATOR: With a closer look via drone,

the shapes appear to be made from thousands of stones.

GUY: The assumption would be that these things, whatever they are,

are the work of people.

But, as we know, nature is also very capable of creating

very sophisticated shapes and patterns.

NARRATOR: Take, for example, another mysterious phenomenon

found in Burnt Cape, Newfoundland, Canada.

CYLITA: These have been called "frost polygons",

and they're made from thousands of piled-up stones

forming these geometric shapes.

DAN: The reason they're called frost polygons

is because they're found in northern or Arctic areas of permafrost.

- Similar kinds of "patterned ground"

can be found in parts of Iceland,

as well as Norway's Svalbard archipelago.

GUY: But they almost look too perfect to be natural formations.

But that is exactly what they are.

NARRATOR: In the colder months, the ground surface freezes and cracks.

And in the warmer months, these cracks fill with water,

which collects and freezes in the permafrost below.

Over time, these ice wedges cause a buckling at the surface

creating these geometric formations.

CYLITA: While Australia's climate wouldn't produce frost polygons,

is it possible that the shapes we're seeing are somehow

created by the environment?

NARRATOR: Another example of nature's creative force

can be found in California's Joshua Tree National Park.

PETER: These are known as "broken terrace walls".

And while they look like they may have been

cobbled together by a stone mason,

these are also natural formations.

DAN: The walls, or "dikes" as they're known,

began millions of years ago

when molten magma pushed up through cracks in the granite stone.

And when it hardens, this molten rock is much tougher

than the granite surrounding it.

Over time, when the rest of the bedrock

in the area gets eroded away,

these strips of tough rock are all that's left behind.

NARRATOR: But the stone formations found across Australia's waters

are made up of individual stones

that seem to have been stacked by hand.

Is it possible they were built by people?

GUY: As the view from above shows us,

humans around the world have been building rock formations

for thousands of years.

NARRATOR: Just south-west of Joshua Tree

is Anza-Borrego Desert State Park,

where the view from above uncovers more striking rock formations.

Dotting the desert landscape are some 500 mysterious stone circles.

DAN: The remnants of these stone formations vary in size from

approximately five to twelve feet in diameter.

NARRATOR: And while this park is nearly the size

of the state of Rhode Island,

these hundreds of circles are concentrated to an area

just over 15 square kilometres.

KAREN: No one is quite sure how old these ring-shapes are,

but it's believed that they were built by Native Americans,

thousands of years ago.

GUY: Now, the reason for making these circles isn't entirely clear,

but past theories have suggested they might have been used as kind of

ritualistic or ceremonial sites.

ANTHEA: But current thinking is that they may have been

used by early hunter-gatherers

as places to rest and take overnight shelter.

CYLITA: And for this reason, they've come to be known, by some,

as "sleeping circles".

ANTHEA: Some have estimated that they could be anywhere from

two to ten thousand years old.

DAN: If people did build these rock formations in Australia,

it seems unlikely that they functioned

as shelters or ceremonial sites,

because they are all offshore or in riverbeds.

- But seeing as how people have been using stone, well,

since the stone age, how old might these structures be?

GUY: So far, radiocarbon testing has proven really quite difficult,

and no one has been able to date the formations

with any reliable measure of accuracy.

CYLITA: But locals in Brewarrina, right next to the Barwon River,

will tell you the stone structures were built by people

a very long time ago.

DAN: The belief is that these formations

could be as old as 40,000 years!

- If true, that would make them some of the oldest-known

human-made structures on the planet!

DAN: There isn't definitive radiocarbon dating,

but archeologists believe that the structures were built more recently.

Probably within the last 1,000 to 3,000 years.

ANTHEA: Either way, these are still very old

with many of them being linked to Pre-colonial Aboriginal communities.

KAREN: But the question remains: What are they?

And what were they used for?

GUY: As we know, people have been building stone formations forever,

and for various reasons. You know,

you have religious and ceremonial sites,

you've got astrological calendars and you've got ancient dwellings.

But there's also another important reason.

NARRATOR: From the waters of Australia to the deserts of Jordan,

the view from above may once again offer a clue.

DAN: This is the Harrat al-Sham,

also known as the Black Desert.

It's a vast volcanic field covering more than 40,000 square kilometres.

GUY: And this black desert is home

to some fascinating stone structures.

KAREN: While they don't look like much from the ground,

the view from above reveals a network

of thousands of these massive stone-built formations.

CYLITA: They've been called "Desert kites"

because their shape almost looks like a children's kite,

especially when viewed from above.

And they're not dissimilar to the shapes in Australia.

DAN: The kites are believed to be Neolithic,

potentially dating back as far as 7,000 BCE.

GUY: They remained a mystery for years,

until researchers finally discovered their purpose.

DAN: They're massive traps.

Used to hunt animals like gazelle for food and for fur.

The low stone walls were built to guide migrating herds

and funnel them straight into the hands of neolithic hunters.

NARRATOR: While these stone-structures in Australia

were built into water,

is it possible that they exist for a similar purpose?

- Now, it turns out that these are also ancient traps!

But these ones are designed to capture fish

and other marine animals.

CYLITA: The Ngemba tribe, that's local to the Barwon River,

refers to these traps as Baiame's Ngunnhu.

KAREN: According to tradition,

the creator Baiame brought these traps into being

by throwing a giant net across the river.

DAN: This is taken from oral history

that the inspiration for the traps' design was taken from nature,

specifically from the pelican and how it scoops fish up

with its large cup-like beak.

- You can see this mirroring of nature,

in that many of the traps are built in these cup-like U shapes.

DAN: And the U shapes are all facing downstream

which would seem counter-intuitive if you're trying to catch fish.

But that's because the fish here are swimming upstream.

ANTHEA: So the fish swim upstream, guided by the stone walls.

And then they land in these holding ponds.

And it's here that the fish could be speared, or even caught by hand.

DAN: Rocks could also be placed strategically.

You could block or unblock specific areas

and channel the fish wherever you want.

- The beauty of these traps is that they could be kept in place forever,

and when the gates were open, fish could swim in and out freely.

- They were also designed in such a way as not to inhibit

the natural flow of the river.

CYLITA: Based on local history and tradition,

it's apparent that being responsible stewards of the river

was extremely important.

NARRATOR: But what about the similar formations

seen from above the country's shorelines?

How did they work?

DAN: The traps found along Australia's coasts,

these were designed to use tidal fluctuations to achieve a catch.

ANTHEA: The stone walls are built low enough

so that the fish can swim into the trap at high tide,

but also high enough so that some of the fish get trapped

in these stone-pens when the tide goes back out.

CYLITA: These Australian stone-walled traps

are a feat of engineering.

And their traditional use is

an example of sustainable fishing practices

created in harmony with nature.

DAN: From the ground,

many of these ancient traps would be totally missed.

It's only when you look at them from above that you can fully appreciate

their scale and the ingenuity of their ancient creators.

NARRATOR: From sprawling stone fish traps in Australian waters,

to a yellow brick road on the ocean floor;

an underwater cemetery

and luxurious Roman ruins -

kept hidden beneath the waves.

All hold keys to the past.

Mysteries uncovered with a view from above.

Subtitles by Sky Access Services