Original subtitles

- (birdsong)

NARRATOR: An aerial view reveals an ancient puzzle.

- Who built it? And why?

NARRATOR: A battlefield mystery two centuries in the making.

- There's something really very unusual.

No-one can find the bodies.

NARRATOR: Drone footage captures a startling sight...

- It almost looks like a giant pool of blood.

NARRATOR: ..and otherworldly images from high in the sky.

GEORGE: These dark clouds, glowing red.

I've never seen anything like this.

- (dramatic music)

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?

- (birdsong)

NARRATOR: A birds-eye-view

can provide valuable new perspectives of our world,

often uncovering startling realities that defy expectations.

If it weren't for this unique vantage,

some of the most surprising mysteries

might remain hidden right under our noses...

or more accurately, our feet.

In the American Midwest, in the town of Newark, Ohio,

lies one such site.

And it happens to be in the most unlikely of places.

A private golf course.

- If you look down at the third hole,

you see that the putting green

is surrounded by this large, circular bank.

- It's about six feet high and perfectly round.

NARRATOR: While this obstacle

has been challenging golfers for decades,

it has baffled historians for much longer.

Because the mound actually predates the golf course -

by two millennia.

- This mound dates back to sometime between 100 BCE

and 400 CE,

so it's very old.

- It's clearly not a natural formation.

Somebody put it here.

But why?

- Who would have built this mound so long ago

and how did it end up in the middle of a golf course?

NARRATOR: An even higher look from above begins to reveal clues.

140 kilometres to the southwest, in Peebles, Ohio,

there's another curious earth structure.

NATASHA: Amongst the trees, we start to see this figure appear.

It looks to be man-made, but what is it?

NARRATOR: Spanning over 400 metres,

this formation is known as Serpent Mound.

- It is thought that Serpent Mound was built around 300 BCE

by the Adena Culture,

the name we give to a diverse group of pre-contact peoples.

Serpents were considered supernatural creatures

that were venerated and often depicted

by ancient North American cultures.

- The theory is that Serpent Mound was built as an effigy

of this powerful animal spirit,

and was likely used as a ceremonial site.

- As far as we know, Serpent Mound is the largest surviving

effigy mound in the world.

NARRATOR: Was the mound at the golf course built for a similar purpose?

Could it be a ritualistic place

dedicated to a powerful animal spirit, god, or other deity?

SHEILA: The circle doesn't seem to be a figurative representation

of a specific animal or being.

But maybe we're missing something.

NARRATOR: A view from satellites above

starts to reveal a bigger picture.

- When you zoom out and take a look at this area,

you see that there isn't just this one circle.

NARRATOR: A short distance from the third hole

is another circular mound.

Only, this one is much larger.

SHEILA: The second circle is over a thousand feet in diameter

and encompasses an area of 20 acres.

And it also seems to connect to a group of straight mounds.

NARRATOR: In total, there are eight of these linear mounds,

each 168 metres long.

And surprisingly, when viewed from above,

they form the walls of a giant octagon.

ROMA: The Octagon spans 50 acres.

That's over two million square feet.

To put that into perspective,

you could fit the Roman Colosseum into it four times.

NARRATOR: This massive structure has been called the Circle-Octagon.

And this isn't the only geometric shape in the area.

Almost two kilometres southeast of the golf course

is another giant circle.

ROMA: This circular enclosure is larger than the others.

Fittingly, it's been called The Great Circle.

NATASHA: Given the size and scale of these mounds,

this site would have had a huge cultural

or religious significance to whoever built it.

NARRATOR: All over the world,

we find evidence of mysterious ancient sites,

from the famed Stonehenge in rural England,

to the Mayan ruins of Latin America.

Is this site in the United States

the remnants of another great and ancient civilisation?

Archaeologists in the area had the same question.

When they began mapping these mounds in the 1840s,

what they found was astounding.

- The maps clearly show the Circle-Octagon

and The Great Circle.

But they also show that these formations

were actually a part of a much larger network.

NARRATOR: Superimposing these early maps

onto current satellite images

shows just how expansive these earthworks once were.

SHEILA: The maps show long, connecting corridors

and something called "The Square".

Its footprint was almost twice the size

as that of the Great Pyramid of Egypt.

What these maps clearly indicate

is that this city was built on top of one of the largest known

geometric earthwork complexes in the world.

- It must have been a place of extreme significance,

because it would have taken an enormous number of workers,

well into the thousands, to complete this.

And the work likely spanned multiple generations.

KAREN: Even with today's modern machinery,

that kind of construction project would be no small feat.

But 2,000 years ago, they would've had only the most basic of tools.

NATASHA: Archaeologists have found primitive hoes in the area,

made from clam shells.

Imagine building these structures with tools made from shells,

wooden sticks, and animal bones.

It's amazing.

- They would have likely moved all of that earth

with handwoven baskets, one by one.

NARRATOR: Sadly, much of these earthworks

have been destroyed over time.

But certain fragments were adapted and maintained for modern use,

including the section that was incorporated into the golf course.

- But the question remains: Who built it and why?

NARRATOR: Two distinct markings found on the original maps

may provide the answers.

- The map-makers indicated a path, or route of some sort,

that branched off from the Octagon, heading south.

- This path was made from elongated mounds.

But this wasn't a small footpath.

It was an avenue 200 feet wide.

- Could this be evidence of an ancient travel route of some sort?

- But who might have travelled these ancient routes 2,000 years ago?

- They would have been hunters and gatherers,

and also farmers that lived in a collection of small communities.

SHEILA: There was a group of people that lived in the region

between 100 BCE and 500 CE.

They are referred to as the Hopewell Culture.

NARRATOR: It is believed that the heartland of the Hopewell Culture

was located 96 kilometres south of the golf course,

in Chillicothe, Ohio.

Could the route indicated on the map

have once linked the Newark earthworks

to settlements in the south?

Many believe this to be the case, that members of the Hopewell culture

were the architects of these massive geometric earthworks.

A legacy that is recognised and celebrated

by Native Americans today.

- These were built by my ancestors, by my forefathers.

What we have today is spectacular.

We have only pieces left, but try to imagine and try to fathom

what it would've been had it been preserved and kept.

When you look at these walls, you can tell the immensity of it.

That's seven million cubic yards of soil.

The knowledge that had to go into it,

the enormity of the design.

They had to share that common vision,

that common desire to accomplish this.

- But what are these structures for? And why build them here?

NARRATOR: At first glance, these earthworks appear to be

a collection of random lines and shapes.

But with a view from above, certain patterns emerge.

- The size and orientation of these mounds aren't accidental.

- For example, if you look at the Circle-Octagon,

you could draw parallel lines

from the axis points of the two circles a mile apart.

The corners of the Octagon and the corners of the Square

also join up with perfectly parallel lines.

NARRATOR: And researchers began to wonder whether

this earthen geometry might correspond to patterns above.

- (animal howls)

- We all know the moon goes through phases

over the course of about a month, right?

Every 29-and-a-half days, you get a full moon.

But the moon goes through other cycles, too.

Look at where the moon sets on the horizon.

There are these eight points where the moon seems to reverse direction

over this thing called the 18.6 year cycle.

Incredibly, all eight of those points

are represented in the geometry of those earth mounds.

And the moon's complex patterns culminate in this thing

called the extreme northernmost moonrise,

or the major lunar standstill.

It only happens once in the whole cycle, every 18.6 years.

And guess what? When the moon is in this position,

it aligns perfectly along the centre axis of the Circle-Octagon.

NARRATOR: Is this golf course in Newark, Ohio,

sitting on top of the world's largest lunar calendar?

Could it have been a place for the Hopewell Culture to gather

and bear witness to the moon's rare standstill?

SHEILA: It's likely that people came to this site from all over

to witness what was probably a once-in-a-lifetime event.

The journey here was likely as important to the Hopewell Culture

as other great pilgrimages the devout make today.

GLENNA: For 2,000 years, people gathered here.

This was a central religious place, a sacred place.

This place was built because people believed in it,

because there was a power, because there was a spirit here.

That spirit, that power, it still exists today.

With Native Americans, unfortunately,

the word "savages" seems to be the historical context that we have.

When you stop and realise the complexity of it,

it could not have been savages that did this.

They were geniuses.

NARRATOR: And while many of these earthworks

have been lost over time,

aerial technology paints a stunning new picture.

Back in Chillicothe, Ohio,

modern LiDAR imaging detects something extraordinary -

another almost identical circle-octagon.

- And this one was built on an exact 90-degree angle

to the one at Newark.

NARRATOR: All this suggests that the two sites were in fact connected

in a much larger network of geometric earthworks.

- It's an engineering wonder that can only be fully appreciated

with a bird's-eye view.

And that's a vantage we're lucky to have today.

NARRATOR: While a view from above

can showcase humanity's great capacity for cooperation,

it can also expose our propensity for conflict.

In Belgium, just south of Brussels,

is the site of another massive earth mound.

And this one surrounds a shocking military mystery.

GUY: This is Lion's Mound,

built to commemorate the Allied victory over the French

in the world-changing Battle Of Waterloo.

The French Emperor, Napoleon Bonaparte,

was attempting to conquer the rest of Europe,

but it was at the Battle Of Waterloo in 1815 that he lost,

and that puts an end to his reign and to France's dominance in Europe.

- The battle only lasted a day, but it killed thousands of people.

- There's something really very unusual about this battlefield

because despite the massive loss of life,

no-one can find the bodies.

NARRATOR: Some mysteries are less about what can be seen

and more about what's missing.

Over the centuries, Europe has served as the theatre

for countless battles, not to mention two world wars,

the cost of which is apparent when viewed from above.

- Europe's former battlefields are filled with war graves.

These are a truly grim reminder of the cost of war.

GUY: It's just row upon row upon row of headstones.

And when you look at it from above, that gives you a whole appreciation

for this staggering, shocking loss of life.

NARRATOR: But the view above Waterloo

is not what you might expect.

- When you compare this site to other European battlefields,

you can't help but notice

the conspicuous absence of grave markers.

- What happened to all the soldiers who lost their lives here?

NARRATOR: Next to Lion's Mound, in Waterloo's memorial museum,

a macabre relic may offer answers.

Encased in glass is the skeleton of a soldier

who fought and died at Waterloo.

- This skeleton was unearthed quite recently, only in 2012.

And this soldier was found with a musket ball inside his ribcage,

indicating that he was likely shot and succumbed to a punctured lung.

- Now, there's actually something really quite special

about this skeleton.

Because this is the only body to have ever been recovered

from the battlefield at Waterloo.

DAN: Thousands died here in one day,

and only one body has ever been found.

How is that possible?

NARRATOR: But there's a group of battlefield sleuths

determined to dig up clues.

Waterloo Uncovered is an international team

of volunteers, military veterans, historians and archaeologists

working to unravel the mystery of what happened at Waterloo.

- We're looking here for anything related to the battle,

to help explain this mystery of what happened to the dead of Waterloo,

which is a very, very significant historical question.

NARRATOR: On their quest for answers,

team uses geophysical instruments

that could help them find lost bodies.

DUNCAN: What we're looking for is any sort of magnetic anomaly

that stands out from the background.

From that point, we don't know

exactly what the source of that anomaly is.

So in order to figure that out, we have to take a sample

and hopefully explain what's causing that magnetic anomaly.

We've got a pretty dense concentration of charcoal,

little bits of mortar as well, it appears.

We've determined this is more likely to be a pretty modern feature,

so we'll just photograph it, record it

and move on to the next anomaly of interest.

- (tense music)

GUY: If you look at the many battlefield cemeteries

across Europe, you can see that immense efforts have been made

to honour the dead and to try to account for

each and every soldier that was killed.

It is not until the First World War where you start seeing

memorialisation, commemoration on an individual level.

NARRATOR: But a century before World War One,

the Battle Of Waterloo belonged to a much different era.

- For much of history, it was common practice

for armies to just leave the fallen behind.

GUY: These soldiers typically belonged

to the lowest strata of society.

When you have overwhelming numbers of dead,

what happens is the bodies are often just hastily buried

or just left to rot.

NATASHA: But the fact remains, thousands died here.

You'd expect to find some trace of them,

especially with today's technology.

NARRATOR: It's a mystery that has people searching

over two centuries later.

But that can be difficult 200 years after the fact.

- If we examine other battlefields from the Napoleonic era,

we begin to see clues as to what might have happened at Waterloo.

NARRATOR: Just northeast of Vienna, Austria,

is the village of Deutsch-Wagram,

the site of another deadly Napoleonic battle.

- At the time, the Battle Of Wagram

was the largest battle in European history.

And just like at Waterloo,

tens of thousands of men were killed there.

But there's one major difference, and that's what's been found.

NARRATOR: In 2018, archaeologists working at Wagram

made a gruesome discovery - several skeletons

buried just under the topsoil of a farmer's field.

- Over 50 bodies have been excavated from the area.

They determined that these bodies

belonged to soldiers who fought there.

NARRATOR: And it was found that these soldiers

were clearly buried in mass unmarked graves.

KAREN: The use of mass graves was commonplace at the time.

And archaeologists are finding evidence of this,

not only at Wagram but at other battle sites of the time.

NARRATOR: Despite extensive searches,

no mass graves have been found at Waterloo.

Is it possible that we've been looking in all the wrong places?

Could the answers be hidden within Lion's Mound itself?

- The mound is massive.

When it was built, they moved 300,000 cubic metres of soil

from the surrounding area, and some have wondered

if all this soil doesn't contain the remains of the fallen.

NARRATOR: Could this symbol of victory

have also been built to double as a massive tomb?

- The monument was completed about a decade after the battle,

so its purpose wasn't to serve as a container for the dead.

But if there were remains in the surrounding area,

it's conceivable that some of them

may have been incorporated into Lion's Mound.

But it still wouldn't account for all the missing dead.

NARRATOR: Nearby, in a ditch next to the former battlefield,

Waterloo Uncovered makes a rare and unnerving discovery -

the remains of three separate legs.

But curiously, these legs aren't accompanied by full skeletons.

- This particular location was the site of a field hospital.

And contemporary war diaries estimate that, during the battle,

some 500 emergency amputations took place here.

And these leg bones are evidence of that.

NARRATOR: And more surprising evidence is found,

including the complete skeletons of horses.

- When we think about the casualties of war,

we forget that thousands of horses

have also been killed as a result of human conflict.

NARRATOR: Along with these horse skeletons,

the group finds something else they didn't expect -

a complete set of human remains,

the second body ever found at Waterloo.

- The story of this particular soldier remains unclear,

but since his body was found so close to a field hospital,

he may have died on the operating table,

with his remains dumped into a pit

used to dispose of amputated limbs and euthanised horses.

NARRATOR: If this soldier was put here

by the people who worked at the field hospital,

who dealt with the thousands of others who died on the battlefield?

- Battles like this would leave the countryside

littered with the carnage of war,

and the mess was often left for the local people to clean up.

Records show that after the Battle Of Waterloo,

dozens of local peasants were responsible for this unenviable job.

- But before soldiers' bodies were disposed of,

they were stripped of valuables and weapons.

- This sort of scavenging was rampant.

Clothing, fabric, shoes...

these were highly valuable commodities

to all the poor at that time.

NARRATOR: But that wasn't the only thing taken from these soldiers.

At Waterloo, the act of battlefield scavenging

reached a grisly new level.

- People used pliers to actually wrench the teeth from the dead

and they then sold those teeth abroad to make dentures.

And these were known as "Waterloo teeth".

NARRATOR: After all the pillaging and pulling of teeth,

what happened to these casualties of war?

- As far as we know, the majority of the bodies were disposed of

in much the same way as other battles of the time -

buried in mass graves, similar to the ones discovered at Wagram.

NARRATOR: And one likely site for such burials is Hougoumont farm,

a place of fierce fighting during the battle.

Historical images depict the use of mass graves in this very location.

- You have one painting with mass graves,

with mass graves under construction.

We opened three big trenches over there

and we didn't find anything like that.

NARRATOR: And yet historical accounts suggest

that this site should be rife with bodies.

- So there on the left, you have what is called the "Killing Ground".

That stripe of land was covered with bodies, mainly French bodies.

But we excavated major trenches over there and nothing,

absolutely nothing.

NARRATOR: Why, despite graphic historical records

and a widespread search, have so few bodies been found?

- There are those who believe the reason

no-one is finding these mass graves is that the bodies were taken.

KAREN: There's a strong argument to be made

that the bodies are no longer in the ground,

and the explanation is stranger than you might think.

- (birds caw)

NARRATOR: The search for answers may lead us beyond the battlefields

to an unexpected place -

the vast farmlands of the British isles.

- Prior to 1840, there was no such thing as chemical fertiliser.

Before that, farmers often fertilised their crops

using something called bone meal.

Fertiliser companies would make this bone meal -

or "dust" as they called it - by crushing up animal bones.

And one of the biggest markets for this raw material

was the British Isles.

- We have historical proof that fertiliser companies

raided other Napoleonic battlefields,

where they scavenged the bones of horses and of fallen soldiers.

These bones were then shipped to bone grinders in the UK.

NARRATOR: The concentration of phosphorus found in bone mineral

makes it an ideal fertiliser.

It's believed to be one of the reasons

poppies flourish in Belgian fields,

fed by the remains of thousands of soldiers

whose bodies were never recovered.

- Now, it's been estimated that more than a million bushels

of human and animal bones were imported to Britain

from continental Europe at the time.

- Bones were big business.

And so it's believed that "bone procurers"

raided the battlefields in the years after the battle.

- It's almost hard to grasp the idea that these mass graves

were potentially used as bone quarries.

But locals easily could have shown bone traders to the burial sites,

since many of them helped bury the bodies in the first place.

NARRATOR: While historical records suggest this practice occurred

at the sites of other Napoleonic battlefields,

there's no direct evidence to prove

that bones from Waterloo were used as fertiliser.

If answers aren't to be found in the fields of the British Isles,

is it possible that the crops surrounding Lion's Mound

may hold the key?

- This part of Belgium was known for producing sugar beets,

which are among the most common plants used to produce white sugar.

To make refined sugar, these beets were juiced,

and that juice had to be filtered through something called bone char,

which, as the name suggests, is also made from ground-up bones.

- And about 20 years after the battle,

you have this huge sugar factory being built,

and that's less than three miles from Lion's Mound.

- If you look at old illustrations of the factory,

you can clearly see Lion's Mound in the background.

- Around the same time, trade in bones took off,

when lawmakers liberalised the practice.

And there are many written accounts

suggesting the bones of thousands of horses and countless men

were taken from the battlefield

in order to fuel the country's growing sugar industry.

DUNCAN: In my opinion, the sugar beet theory is a strong one.

Now, in order to either support or refute this theory, of course,

we'd have to find physical evidence of either an intact mass grave,

or on the other hand, a grave but it's been emptied out.

This is the sort of thing we would be looking for

to lend credence to these theories.

NARRATOR: There's no telling what other secrets may be hidden

deep within these fields.

But if more bodies remain out on the battlegrounds -

ones that may have evaded the bone collectors -

the team of Waterloo Uncovered

is determined to continue their search from above.

NARRATOR: Just when we think we've seen it all,

a view from above can reveal something completely unforeseen.

2,000 kilometres from Lion's Mound,

another mystery is waiting to be solved.

This time in the Saronic Gulf of Greece.

- This beautiful landscape,

it's dotted with these picturesque islands

and beautiful white, sandy beaches.

NARRATOR: But in 2021, drone footage reveals a jarring image.

In the centre of the gulf, on a small islet called Metopi,

is what appears to be a red lake.

- This red water looks completely out of place.

- It almost looks like a giant pool of blood.

Like we found some kind of crime scene.

- It's really strange.

Why is this lake red?

NARRATOR: Potential clues may be found

240 kilometres to the north,

in a Greek village famous for thermal hot springs

that just so happen to run red.

- The name of this village, "Kokkino Nero",

is Greek for "red water".

NARRATOR: Greece has more than 700 natural hot springs,

and many of them are believed to have healing powers.

- And people travel from all over

to bathe in the red water of Kokkino Nero.

- But most of the country's hot springs are not red,

so there's something special about this spot.

DAN: The springs get their colour from iron oxides in the water.

It's rust. So iron, oxygen, water,

they're all coming together to make this reddish-brown reaction,

and that's why you get these iron hydroxides

turning the rocks and hot springs red.

NARRATOR: Could the red water on Metopi be fed by a similar source?

- Greece does have hundreds of known hot springs,

but this water on Metopi is not one of those.

- No-one comes to bathe or cure their ailments in this water.

NARRATOR: While some red waters, like those in Kokkino Nero,

are believed to heal, there are others that have proven deadly.

Over, 10,000 kilometres away,

at San Angelo State Park in West Texas,

lies another mysterious red lake.

- Officially it's called the OC Fisher Reservoir

but it's better known as the "Texas Blood Lake".

- It seemed like one day these were healthy waters, with great fishing,

and then the next, it was blood-red, filled with thousands of dead fish.

- People from various religious communities saw this

as a sign of end times.

And you have to admit, it does evoke images of the apocalypse.

NARRATOR: What happened to these once-thriving waters?

And could something similar have impacted the island of Metopi?

- The case of the OC Fisher Reservoir started with a drought,

leaving it almost completely dry.

What little water was left became stagnant,

and this is when it started turning blood red.

NARRATOR: This stagnant lake became a breeding ground

for microorganisms that flourish in oxygen-deprived waters.

- These microorganisms belong to a family

known as the purple sulphur bacteria,

which accounts for the reddish hue we're seeing.

NARRATOR: But Texas' Blood Lake isn't the only red water

that would seem to signal the end of times,

nor is it the strangest.

In Tanzania, on the African continent,

there's another example of deadly red waters.

This one even more terrifying.

- If you look at this part of Tanzania via satellite,

it's unmistakable.

- You see this massive body of deep-red water.

NARRATOR: This is Lake Natron,

and its waters aren't the most hospitable.

- First off, the water is hot,

at times, reaching up to 140 degrees Fahrenheit.

To put it in perspective,

that's about 50% hotter

than you'd ever want in your hot tub.

- But what makes this water so lethal isn't the temperature,

it's the mineral and chemical makeup.

- Lake Natron is close to an active volcano called Ol Doinyo Lengai,

known as "The Mountain Of God", and it's a very strange volcano.

It's the only volcano on Earth that spews this dark black lava

called natrocarbonatite, or "natron" for short.

It turns out it's the coldest lava in the world,

but it will still burn you.

NARRATOR: This strange lava formed the bedrock in the area,

giving the lake its unique composition.

- Lake Natron is highly basic or what is commonly known as alkaline.

The pH here can match that of ammonia,

so it's incredibly corrosive and dangerous.

- Similar to Texas' Blood Lake,

Natron has been turned red by microorganisms.

In this case, it's an algae containing red pigments.

And this algae actually thrives in this extreme environment.

- So the lake's chemical composition,

coupled with this algae, has created this caustic red brine.

NARRATOR: This noxious water will quickly burn the skin and eyes

of most animals that come into contact with it.

- In the dry season, when the lake recedes,

you can see all this carnage that's been hidden below the surface.

But Lake Natron's red water doesn't just kill unsuspecting animals.

In some cases, it mummifies them.

- Several species of birds, bats,

and other animals have been found petrified.

- If the Texas Lake of Blood is like something out of the Bible,

this is more like something out of Greek mythology.

It's like these animals have seen Medusa.

DAN: This lake prevents decomposition

because it turns out that the chemicals in it are almost identical

to the ones used by ancient Egyptians for mummification.

NARRATOR: While it's a death trap to most animals,

the red water of Lake Natron

has allowed some rare species to flourish,

including 2.5 million lesser flamingos.

- These flamingos have tough, scaly legs

that protect them from being burned by the lake's caustic waters,

and they even have these special glands in their nasal cavities

that help them to filter out salt from the water.

- Lesser flamingos also feed off the algae in the lake.

And it's the same algae that give the water its red colour

that give the flamingos their pink plumage.

NARRATOR: Could colourful algae or other microorganisms

explain the red water discovered in the Saronic Gulf?

For now, the answer remains murky,

but the view from above reveals another puzzling image -

a cryptic inscription emerging from the bottom of the red lake.

- OK, this just got even stranger.

- Someone has definitely written something

at the bottom of this lake.

It looks to be three horizontal lines,

then an "X" followed by another three lines.

- These look to be letters from the Greek alphabet.

SHEILA: The three lines do resemble

the 14th letter in the Greek alphabet.

It's called the letter "xi".

The "X" could be the Greek letter "chi" followed by another "xi".

- This doesn't spell anything.

Could it possibly be initials or maybe an acronym for something?

- Could someone have buried treasure on this island?

Does "X" really mark the spot?

- Or could this be a simple land marker

placed by whoever owns the land?

NARRATOR: But the question of who owns this land

isn't an easy one to answer.

- For years, the island was used for small-scale farming

by people from larger, neighbouring islands.

And those people all apparently

owned and occupied small plots of land.

- And so deeds to hundreds of these small plots

were handed down from generation to generation.

NATASHA: And it seems that things may have become fairly acrimonious

amongst the current owners.

None of them can agree on what to do with the land.

NARRATOR: Perhaps the peculiar symbol was put there

as a claim to land, or even a warning.

But how is it that it comes to lie at the bottom of the lake?

CYLITA: If you look at the island via satellite,

the water isn't always there.

At times, it almost seems to disappear.

- This is a shallow lake, and under the hot Mediterranean sun,

it makes sense that it would sometimes evaporate.

So someone could have easily made this mark when the lake was dry.

- So when you pay attention to the water over time,

the thing that you'll notice is that this lake is always changing.

Sometimes it's red, and other times it's milky white.

You know, this change is almost identical

to what we see in the salt flats of San Francisco Bay.

NARRATOR: Could clues to the lake's colour

be found on America's west coast?

Spanning more than 16,000 acres,

the San Francisco Bay salt ponds are used

in the commercial harvesting of salt.

- These ponds are massive, and sometimes they're bright red,

and at other times they're more white.

- Salt producers gather salt water in these shallow ponds

by basically just evaporating all the water off

so that the salt concentration increases.

And, as that happens,

you get the perfect environment for salt-loving algae.

- And in order to defend themselves from the sun's UV radiation,

these algae contain high levels of beta-carotene,

which is this orangey-red pigment.

So the cycle of salt water pooling, evaporation,

followed by algae growth is why we see this change in colour.

And that's what could be happening here on Metopi.

Perhaps it's a naturally occurring salt pond.

GEORGE: It makes sense.

Waves would push salt water onto the island's shores,

then the pooling water evaporates, increasing the salt concentration.

This is the perfect environment for red algae to bloom,

creating this red lake.

- All of these changes are happening on the microscopic scale,

and yet you can see them from way up above.

- As for the symbol in the lake... that's still a mystery.

NARRATOR: From a small, uninhabited island to vast ocean waters,

the view from above can reveal perplexing sights,

and often when you least expect it.

In 2022, a commercial pilot flying high above the Pacific Ocean

captured cellphone images of something very unusual.

- These dark clouds glowing red. I've never seen anything like this.

GUY: It looks really otherworldly.

It kind of looks like a scene from War Of The Worlds.

NARRATOR: The origin of this glowing is unclear,

leaving the pilot and crew to theorise

over a possible explanation.

- When these images were posted online,

the internet blew up, with people theorising about everything

from secret military operations, even UFOs.

- Before we make the leap to visitors from outer space,

let's rule out some more earthly explanations first.

This particular flight was fairly high up in the Northern Hemisphere.

So could this maybe be connected to the Northern Lights?

CYLITA: They're called the Northern Lights

because they appear in the Earth's Northern Hemisphere.

The closer you are to the magnetic pole,

the better your chances of seeing them are.

GEORGE: While the Northern Lights are most typically green,

the colours can vary depending upon atmospheric conditions,

and at times they do appear red.

NARRATOR: Could the Aurora Borealis explain this red luminance

seen from the cockpit?

There's one problem with the theory.

- Typically, the lowest part of the Aurora

sits about 300-000 to 400,000 feet up.

This plane was flying much lower, at around 31,000 feet.

So while you might see the Northern Lights along the horizon

or up higher in the sky, you wouldn't see them

clustered like this, directly below the plane.

NARRATOR: If the Aurora Borealis isn't the answer,

could we be witnessing the effects of another celestial phenomenon?

- When I see these images,

they almost look like a Transient Luminous Event.

- Transient Luminous Event,

now that's a term for upper atmospheric lightning,

and one of the forms it can take is something called a red sprite.

- Red sprites are huge, sometimes stretching as long as 31 miles.

- And the reason they're red is due to molecular nitrogen

emitting low-frequency red light when it gets excited.

- One of the reasons they're rarely seen

is because they occur in the atmosphere above thunderstorms.

So there usually isn't a clear line of sight from the Earth.

- One thing, though, is that these sprites travel really fast.

They only appear for three to five milliseconds.

That's barely enough time for the human brain

to even register seeing them.

It's nothing like what we see here that's lasting several minutes.

NARRATOR: While this red light seems to be anomalous,

it isn't an isolated occurrence.

Another pilot, this time travelling from Hong Kong

to Anchorage Alaska,

makes an eerily similar discovery.

- In 2014, you get a different pilot capturing images

that seem to show exactly the same red glow.

It's really uncanny.

- And this pilot makes his images public as well,

because he also has no idea what he's seeing.

NARRATOR: Images captured eight years apart,

both showing the same bizarre phenomenon.

Could the two events be related?

- Now, when you look at where each of these aircraft were

when they saw this red glow,

they were both flying southeast of Russia's Kamchatka Peninsula.

NARRATOR: Could something specific to the region

be causing this strange phenomenon?

- The Kamchatka Peninsula is part of the Pacific Ocean's

infamous "Ring Of Fire".

That's home to three-quarters of the world's active volcanoes

and this particular area is a known hot spot.

- They call it Russia's Yellowstone, and that is for good reason.

- This area has got at least 29 active volcanoes.

- Were these pilots unknowingly flying over a volcanic eruption?

- Volcanic islands do exist in the Pacific Ocean,

but we're not seeing the plumes of volcanic ash

we'd usually associate with an eruption.

NARRATOR: In 2010, a volcano in Iceland

ejected massive plumes of volcanic ash,

resulting in what, at the time,

was the largest shutdown of air traffic since the Second World War.

GEORGE: Volcanic ash is a major safety hazard in airspace.

So if there was an eruption somewhere inland

or a volcanic island in the ocean happens to go off,

these planes would have been rerouted.

NARRATOR: But not every volcanic eruption

fills the sky with large clouds of ash.

- It's estimated that 80% of volcanic eruptions

actually take place under water.

NARRATOR: Is what we're seeing the result of an underwater volcano?

- Unlike volcanoes that erupt on land,

underwater volcanoes don't tend to have the same explosive eruptions.

The pressure of the water from above

keeps the lava close to the seafloor,

creating something called a "passive lava flow".

- Also, when the lava makes contact with the cooler water,

it quickly forms a solid crust.

So it would be unlikely to see such a large and persistent red glow

on the surface of the water, let alone from 31,000 feet up.

NARRATOR: If the area's scores of volcanoes aren't the culprit,

what else might link the phenomenon

to this particular geographic location?

- Besides being one of the most seismic places on Earth,

the Kamchatka Peninsula has another big claim to fame.

It's home to Russia's largest submarine base.

NARRATOR: Located on Russia's eastern frontier,

the Rybachiy Nuclear Submarine Base

is home to the country's Pacific nuclear submarine fleet.

- This area has been crucial to Russia's strategic defence

and there's long been a heavy military presence here.

- It's not uncommon for the Russian Navy to operate in Pacific waters.

It's their backyard.

NARRATOR: The Pacific Ocean has long been monitored

and used as a testing ground for the world's superpowers.

- Famously, the United States used parts of the Pacific

to test nuclear weapons,

in an area they called the "Pacific Proving Grounds".

NARRATOR: Is this what the pilots happened to glimpse -

a Russian naval fleet running a training exercise

or a weapons test?

GUY: If you look at the scale of this red cluster,

it is perhaps consistent with a really large-scale naval exercise.

And we also know that military fleets,

they're often using red lights to illuminate their ships at night.

But I don't think I've ever seen a glow quite this bright

coming from any naval vessel.

- And these red lights were seen outside of Russian waters.

It wouldn't be standard practice for Russia to have been conducting

unannounced military operations outside their own territory.

- If this were a military exercise or a weapon test,

the airways would almost certainly have been closed

to civilian and commercial aircraft.

So these pilots should not have been able to fly

directly over that kind of activity.

NARRATOR: But a closer look from above

indicates another type of ocean operation.

- Commercial fishing is big business,

and well over half of the world's catch comes from the Pacific Ocean.

- A big target for these fisheries is Pacific saury.

NARRATOR: Saury, also known as mackerel pike,

is a staple in many East Asian cuisines,

and there's huge demand for these fish

in countries including China, Korea, and Japan.

- And one really effective way to catch saury is, well...

to use really bright lights.

NARRATOR: Modern fishing vessels are often equipped

with a large array of LED lights.

When used at night, these lights attract microscopic algae

and plankton that are fed on by saury.

TORRI: So, in essence, these bright lights initiate

an underwater feeding frenzy

that brings the saury up to the surface for the fishers to catch.

- That is exactly what these pilots were seeing.

- It's incredible that these massive red lights

that can be seen from thousands of feet up

are actually designed to attract the attention

of microscopic plankton below.

- Yes, the red glow is definitely a sign of human ingenuity,

but it's also a reminder of the increasing demands

that people are putting on the planet.

NARRATOR: Environmental factors and overfishing have taken a toll

on the migratory patterns and number of saury in the ocean.

In recent years, Japan's saury haul was the lowest on record.

CYLITA: As a result of overfishing,

countless species around the globe are vulnerable,

and our ocean ecosystem is at risk.

NARRATOR: An important reminder that would have largely gone unseen

if it wasn't for a unique vantage from above.

- (dramatic music)

NARRATOR: From a strange symbol etched in red water

to mysterious lights in the ocean...

..and two sets of earthen mounds an ocean apart...

..the view from above can provide unexpected insights

into our collective past, present, and future.

Subtitles by Sky Access Services