Original subtitles

[narrator] An uninhabited, muddy little island in England

has a horrifying history.

Authorities believe that there are hundreds of bodies

on the island.

The place is covered in human bones,

skulls, and teeth.

[narrator] A shocking spider discovery on an island off

Sri Lanka reignites an age-old mystery.

- Previously, they've not found the spider anywhere else

in the world.

Could it have traveled by air?

[narrator] Animal corpses discovered on the shore

of a small Canadian island baffle researchers.

At first glance, the wounds appear intentional and precise.

What could be ravaging these poor creatures?

[narrator] Isolated, scarce on resources, islands are worlds

unto themselves.

Bizarre creatures, ancient gods, and haunting ruins.

Baffling murders and deadly spirits.

What will be discovered on Earth's mysterious islands?

♪ ♪

[narrator] The British Isles, over 6,000 islands

that have been intermittently inhabited

since as far back as 800,000 BCE,

the largest of which are Great Britain and Ireland.

You probably don't know the names of the smaller ones,

the ones that aren't much more than uninhabited stretches

of marsh, mud, and rock.

But if you do find yourself on one

that's barely above sea level, you'll also probably find

lots of stuff in the mud, like old bottles, plastic

containers, rubber boots.

And every once in a while, people have even found

shrapnel from bombs that struck the region

during the Blitz.

[narrator] But on one small island, just about 40 miles

from London, you're likely to see something

far more mysterious than shrapnel.

In 2016, people sneaking onto an off-limits avian

wildlife preserve found themselves surrounded

not by birds, but by bones.

And these were not the bones of wild animals.

The place is covered in human bones, skulls, and teeth.

If you're actually walking on the island,

there's an excellent chance of you literally

tripping over them.

What happened here?

Authorities believe that there are hundreds of bodies

on the island, which isn't really a surprise

when you realize that the name of the island

is a bit of a spoiler, Deadman's Island.

Why are there so many human bones here?

Could this be one of those plague islands

you hear about, like Poveglia?

[narrator] When the bubonic plague arrived in

Europe in 1347, the small Italian island of Poveglia

became a quarantine site for many

dying from the Black Death.

Even hundreds of years after the plague,

the island remains off-limits to visitors.

While both Poveglia and Deadman's Island

are closed to the public, that is sort of where

the similarities end.

Poveglia has a lot of buildings, housing, a bell

tower, a hospital, and it even had a fortress.

Deadman's Island is less than a mile long

and just 650 feet wide.

It has almost no structures.

It's really just mud and marshland.

Where would people live or even die here?

If you walk across the island when the tide is out,

you can see even more bones jutting out of the mud,

some much smaller than others.

They're clearly from children or young adults.

Why would anyone be buried here, let alone children?

Despite the menace of the name,

this uninhabited mud bank is currently a

protected wetland site and a bird breeding

and nesting preserve.

But long before it was an avian love island,

it had a much darker purpose.

[narrator] In the 1700s, the British prison system was

nearing collapse and part thanks to the

Waltham Black Act of 1723,

which we now know as the Bloody Code.

It was an emergency measure to punish crime.

It included serious crimes like arson and murder,

but it also covered a wide range of thefts

we might think of as being pretty minor.

But in that era, there wasn't much of a distinction

between the petty crimes and the serious stuff.

♪ ♪

Basically, if a 10-year-old stole a chicken

or a loaf of bread to survive, they could find

themselves in jail.

[Anthony Cantor] As a result of all these draconian laws,

prison populations swelled until the prisons couldn't

accommodate any more people.

So the authorities came up with the novel idea

of floating prisons called prison hulks.

[narrator] A hulk, in the language of the British Navy,

was a ship that was no longer capable of full service.

Such ships were gutted of all their military gear

and turned into prison quarters for anywhere from

100 to 600 people.

On the prison hulks, some of the prisoners were

to be there indefinitely.

Others were just held until there was a merchant ship

big enough to bring them to colonies like Australia

for forced labor.

Of course, we know prisons don't offer

the most pleasant of accommodations at

the best of times.

If you now imagine all of that crammed

onto a dark, stuffy ship, it's even worse again.

Apparently, the hulks were far worse

than even the Dickensian prisons.

The quarters were crammed and filthy,

and the food and water were often contaminated

with bacteria.

[narrator] Cholera hit Britain in the 1830s,

devastating the country in 1832, and the prison hulks

were no exception.

Cholera is transmitted by fecal matter.

Imagine how quickly that disease would spread

in an enclosed space with no proper sanitation system.

So when prisoners were infected,

they were moved to the island and left there to die,

which explains the bones, but it doesn't explain

everything on the island.

[narrator] One of the many mysterious things about

Deadman's Island is that there are coffins on it.

In fact, the place where most visitors land

is called Coffin Bay.

You can see the rotting coffins

and pieces of wood all over the place.

[narrator] Now with erosion and rising water levels,

these bodies are also rising up, but it's not the first time

their stories are being heard.

Locals have been telling tales of Deadman's Island

for generations.

Legend has it that there is a devil dog out here

with glowing eyes who eats people's brains.

And some locals swear they can hear wailing

coming from the island at night, a wailing that they say

cannot be dismissed as wind.

Of course, if you find an island covered in bones,

there are gonna be lots of creepy stories about it.

But when you consider that many of these people

had been thrown in jail for things as inconsequential

as stealing a loaf of bread, and that many of the

prisoners were children,

it really does make Deadman's Island

seem much more tragic than terrifying.

[narrator] No one knows what the future holds for this island,

but as the climate changes and water levels rise,

Deadman's Island may soon be washed away,

taking the last of its secrets with it.

♪ ♪

[narrator] Nestled in the Palk Strait in the Bay of Bengal

between India and Sri Lanka sits Mannar Island,

known for its pristine sandy beaches.

- In addition to being beautiful,

Mannar is a haven for the natural world.

There are 3,600 species of plants and animals here,

flamingos, mangroves, and five species of endangered turtles.

It's actually one of the planet's

most biologically diverse coastal regions.

[narrator] With this in mind, a team of Sri Lankan researchers

head to the island in 2012, intending to survey one of

the facets of its diversity, the spider population.

As they catalog the wide range of arachnids,

they encounter something surprising

in the hollow of a tree.

Inside, they find a Rameswaram ornamental tarantula.

Now, finding a spider in a tree when you're studying arachnids

might not seem like a big deal, but think about that name.

Rameswaram is a town in India on Pamban Island.

That is 30 miles away as the crow flies.

And that spider?

Until 2012, it was thought to be endemic,

meaning exclusive to Rameswaram.

Previously, they've not found the spider

anywhere else in the world.

It couldn't crawl over water to the island,

so how did it end up here?

Could it have traveled by air?

Look at a spider.

It has no wings and it can't fly,

so that should be the end of that story, but it's not.

[narrator] In 1832, Charles Darwin, while at sea,

encountered a surprising sight.

The deck of his ship was covered in tiny spiders.

"I caught some of the aeronauts' spiders,

which must have come at least 60 miles.

How inexplicable is this cause which induces

these small insects to undertake their

aerial excursions?"

We should forgive Darwin for calling spiders insects.

In his day, the spiders were considered

a subgroup of the insects.

We now know that's totally wrong,

but the point is, Darwin did witness spiders

traveling long distances in the air,

and since then, they've been documented

two and a half miles above the earth

and going out as far as 1,000 miles out to sea.

[Amma Wakefield] This achievement is both beautiful

and frankly, a little bit terrifying.

Imagine being caught up in a breeze

filled with hundreds or even thousands of spiders.

♪ ♪

But then we're back to the whole no wings thing.

So how do they become airborne?

- Spiders shoot out silky strands to make their webs,

but the same filaments can catch the wind,

allowing the spiders to take flight.

It's called kiting or ballooning.

- Ballooning isn't just the result of a spider's

ingenuity or tenacity.

It's a result of electricity.

[thunder rumbling]

[narrator] Every single day, the earth is host to

electrical storms, which get the air crackling with electricity.

It's thought that these forces can create an electrical

charge imbalance between the silk threads

and the surrounding air.

The idea is that the electrostatic interaction

helps lift the silk strands, taking the spider with it,

hence the term kiting.

♪ ♪

[Alison Leonard] It's a decent theory until

you start thinking about scale.

Most of the spiders you'll see ballooning are pretty slight,

but have you ever seen a tarantula?

These are not small creatures, so it might be pretty

hard to get them airborne.

Plus tarantulas don't even spin those orb-like webs.

They mostly use their silk to line their burrows.

[narrator] So it seems rather unlikely that these large

spiders balloon to Mannar Island.

However, there is another theory about their presence here,

one that's both historical and mythical.

For ships traveling in the region,

the Palk Strait is a bit of a thorn in their collective side.

If you wanna go from the east coast of India

to Southeast Asia, it would make sense

to round the bottom of the continent

and scoot through the Palk Strait

between Pamban and Mannar Islands.

[Anna Klassen] Otherwise, you have to take a circuitous route

and sail to the end of Sri Lanka, only to come up again.

So going through the strait seems like the obvious choice.

The problem is that going through the strait

isn't possible for most ships.

The first challenge is that it's just too shallow in spots,

but that's not the biggest hurdle.

[narrator] There's a rugged underwater barrier

that limits passage through the strait

between Pamban and Mannar Island.

It's a 30-mile-long chain that emerges during low tide.

It looks like a spine.

Since the beginning of this century,

this formation has been the focus of heated debate

between historians, politicians, and religious Hindus,

all arguing about its history, meaning, and utility.

The formation is known as Ram Setu.

According to documents from the nearby

Ramanathaswamy Temple, there was a causeway-like

bridge between these two islands until 1480, when a hurricane

ravaged the region.

[Alison Leonard] A causeway could explain how the tarantula

came to be hiding out in the trees on Mannar Island,

but that theory prompts another question.

Who or what made this causeway?

[narrator] One theory about how this causeway was made

is found in the name itself, Ram Setu.

Ram, or Rama, is a major Hindu deity

and the hero in the Hindu sacred text.

In the Ramayana, Rama's wife has been kidnapped

by the evil 10-headed monster, Ravana,

and taken to Lanka.

In order to save her, Rama instructs Hanuman,

the monkey god, and his monkey warriors

to build a bridge so Rama can reach Lanka and save his wife.

The Ramayana was written by the Indian poet Valmiki,

who lived somewhere in the ballpark of 500 to 100 BCE,

and devout Hindus believe it represents real events

that happened somewhere between 1.7 million

and 3,500 years ago.

[Sarah Klassan] For the faithful, the answer is also found in the Ramayana.

In that text, the stones used to build the bridge

are inscribed with Lord Rama's name,

which essentially enchants them, thus allowing them to float

and be moved into place.

[Amma Wakefield] The reality is there actually is one rock that

floats, and it might be in your bathroom right now, pumice.

[narrator] Pumice is created when volcanic magma

with high gas content rapidly cools and solidifies,

trapping gas bubbles inside.

That results in its porous and lightweight structure

that has a lighter density than water.

Translation - pumice stones can float,

but after a while they take on water and they sink.

So could the magic floating stones of the Ramayana

just be large pumice stones?

[Alison Leonard] The problem with this theory is that there

are no volcanoes in this region, so there's no source

for pumice stone.

What else could have created this causeway?

[narrator] The researchers who found the spiders on

Mannar Island also suspect that there was once a bridge

that connected Sri Lanka and India,

only they believe it was a natural one.

India and Sri Lanka were once connected

and part of the supercontinent of Gondwana.

[narrator] Around 180 million years ago, Gondwana came apart

as a result of shifting tectonic plates, creating continents and

leaving behind land fragments like Ram Setu.

Much of the bridge is determined to be built out of limestone,

which is also called calcium carbonate.

That's the stuff that the exoskeletons

of coral reefs are made out of.

So that's some solid evidence that Ram Setu was

naturally formed.

The debate over how the bridge was made

might seem like an esoteric argument

that only academics and the devout care about,

but it's actually a hot button issue in the country.

[narrator] There have been many proposals to create a shipping

lane canal through the strait, so they would no

longer need to navigate around the bottom of Sri Lanka,

dramatically reducing shipping distances and transit times.

- For Hindus, this is a catastrophic idea.

They believe that this bridge is a tangible part

of their history and their connection

to the gods.

- And it's not just the faithful who are against this plan.

Ram Setu is part of the fragile ecosystem in this region.

It's home to a diverse array of marine life,

and it protects the coastline from strong currents and storms.

- Some scientists fear that the removal of the bridge

could have serious environmental effects on the region,

something especially dangerous in this era

of ever-increasing climate change events,

like hurricanes or cyclones.

[narrator] It's clear that the debate over Ram Setu

isn't likely to be resolved anytime soon.

Thankfully, however the Rameswaram tarantulas

arrived on Mannar Island, they've already made

themselves at home.

The Outer Banks form a thin land barrier

between the Atlantic Ocean and America's eastern seaboard,

formed as the last ice age came to an end.

They have since protected the mainland

from the worst the ocean has to offer.

- The Outer Banks are composed of a number of islands.

One of them is actually among the longest

in the continental US.

This is Hatteras Island, located just off the

coast of North Carolina.

- This relatively flat, sandy spit of land

used to go by another name, Croatoan,

named after the indigenous people that lived here

when the English first arrived back in the 16th century.

[narrator] Archaeologists are looking into the

country's origins, searching for clues as to what became

of the first English colony in America.

This is interesting because as we know,

the first permanent English settlement in North America

was Jamestown, settled in 1607.

But there was another settlement that

predated Jamestown, and that was the vanished

colony of Roanoke.

[narrator] In July, 1587, 118 settlers under the command of

John White landed on the shores of Roanoke, roughly 50 miles

north of Hatteras.

They were making a second attempt

to establish a colony on the island

after a previous colony of the same site

was unable to last a year.

Relations with the local indigenous people

broke down as well.

They had actually killed one of their leaders,

a man named Wingina.

So needless to say, it would have imperiled their colony

as the English were few, vulnerable,

and surrounded by freshly made enemies.

This didn't deter the second group though.

For some time, things must have gone somewhat smoothly

because John White's granddaughter was

born in Roanoke.

She was the first English child born in North America,

and her name was Virginia Dare.

But not too long after they had set up shop in Roanoke,

they realized it was too late in the season to grow crops,

leaving them desperate for supplies.

So John White decided to head back to England to get them.

Problem was, when he got there, the country was in a

full-fledged war with Spain, causing a three-year

delay in his return.

When he finally did come back, all that was left of the

colony were two cryptic clues inscribed into a tree

and a fence post.

The letter CRO and the word Croatoan.

[narrator] Prior to leaving for England, John White had decided that

if Roanoke had to be abandoned for reasons of hardship or war,

the Maltese cross would be carved into a tree.

Thinking that the settlers had simply relocated

to Croatoan, he remarked, "I greatly joy that I

have found a certain token of their being at Croatoan."

Croatoan is the Algonquin name for the island of Hatteras.

So John White figured he would find the remainder

of the colony there.

But terrible weather and a near-mutinous crew intervened,

and he was forced back to England,

never to return to the New World.

The lost colony of Roanoke has become part of the lore

of America's origins.

But what actually happened to it?

[narrator] Under the surface, archaeologists discover

a number of artifacts that speak to Hatteras' rich history.

Artifacts likely made by the Croatoans

are found alongside the firing mechanism of a musket.

They all date to the time of the Roanoke colony.

[Anthea Nardi] There's also a signet ring, a very personal

possession that without a doubt must have belonged

to an English gentleman.

Could these artifacts have belonged

to settlers of Roanoke?

[Alison Leonard] While the ring has been dated to the

17th century, it's very difficult to specifically date

many of the other artifacts.

They may represent a large span of time

or entire different periods.

So we can't say for sure that these artifacts

belong to the Roanoke settlers.

Considering how close Hatteras is to Roanoke,

perhaps the settlers ended up here.

Though given its small size, it's unlikely that the

island could have supported another 100 plus people.

There simply isn't enough evidence

to say for sure either way.

Although a while ago, some rather strange

evidence showed up.

[narrator] In 1937, a man brought a stone to

the history department at George's Emory University.

He said that he found it in the woods

and that inscribed on its surface

was a message from the lost colony of Roanoke.

This is known as the Dare Stone,

so called because the message on the rock

is from Eleanor White Dare, the daughter of John White.

The stone is engraved on both sides

and basically states that both her husband and daughter

were killed in 1591.

And that most of the colony had died

as a result of sickness and war.

- There are several reasons to question

whether or not it truly was written by Eleanor Dare

in the late 16th century.

The message is signed with her initials, EWD.

And for us, this is completely normal.

But for the English in the 16th century, it wouldn't be.

As this wasn't a typical way of signing your

name at the time.

Also, the year 1591 is written using modern numbers.

The practice of using these numerals

didn't really become common until a little later.

[narrator] Geologists have also taken a look

at the stone's properties to see what they can learn.

By cutting off one end, they saw that it was a

bright white on the inside, while the actual words

were much darker.

It would take a long time for that brilliant

white color to fade once the letters had been

etched into the rock's surface.

So this means that when the stone was found in 1937,

it had been lying in the woods for a good while already.

Though a skilled forger could have used chemical stain

to make it a convincing fake.

Regardless of whether or not this message from

the past is a hoax, the explanation offered

by the Dare Stone really isn't that far-fetched.

War, death, and disease would have been a daily reality

for these early colonizers.

There are many different theories

as to what became of the colony.

Some think that it was attacked by the Spaniards,

or that the settlers were either killed

or absorbed into indigenous communities.

If they did abandon the English settlement

and integrate into indigenous communities,

it wouldn't be the first or last time

that European settlers did so.

[narrator] Following the settlement's disappearance,

John White delivered a letter to Richard Hakluyt,

saying that the settlers intended to de-camp

to another location, roughly 50 miles inland from Roanoke.

Exactly where this supposed destination was

has been lost to history.

For unknown reasons, White didn't really

expand on the location.

Maybe he didn't even know himself.

[Alison Leonard] He did draw a map of Roanoke and the

surrounding areas of Chesapeake Bay and Croatoan Island.

But again, on this map, there isn't any reference to

this mysterious destination.

But if you look closely at the map,

you can see that there are faint outlines

that are drawn on top of the original.

These are what we call patches.

They were a common way for map makers at the time

to correct mistakes.

They would simply layer a piece of new paper

on top of the old.

[narrator] Curious about what mistakes White had corrected,

the First Colony Foundation asked the British Museum

to further analyze the map using spectroscopy

and other imaging techniques.

Multi-spectrum spectroscopy essentially allows us

to understand the properties of any given object.

By applying this new technology to this old map,

you can see more than what meets the eye.

[Anthea Nardi] You can actually see beneath the patches.

The bigger of the two revealed a correction

to the coast's topography, but the smaller was

hiding an incredible clue.

[Anthony Cantor] Beneath the little patch at the end of the

peninsula is a four-pointed star, outlined in blue and

filled in with red.

It's located where the Chowan River and Salmon Creek

flow into Albemarle Sound.

It's not much more than 50 nautical miles west of Roanoke.

[narrator] Thinking that there may have been a settlement

on the shores of Albemarle Sound,

between 2012 and 2019, archeological digs were

conducted in the area, indicated by the star

on John White's map.

The area received the name Site X.

What's needed is evidence that could be definitively dated

to the late 16th century.

But this is complicated by the fact

that the first English settlement

in this part of North Carolina was in 1655,

almost 70 years after John White sailed to Roanoke.

So there could be deposits left by someone

other than the Roanoke settlers, if they'd even been here at all.

[narrator] Excavations revealed many artifacts

were made by indigenous people, as well as those

of English origin.

Most importantly, however, a relatively high

concentration of pottery was also discovered,

items that were used by families and regular people,

not the military.

European pottery in itself is not that exciting a find,

but these ceramics do provide something special in this case.

Their production dates can help to establish a timeline.

[Anthea Nardi] The ceramics found at Site X correspond to

the possessions of less than 10 people and were used for food

storage and preparation.

They have the very distinct color that is associated

with what we know to be Surrey Hampshire borderware.

We know that this type of ceramic

was not imported into the New World after 1624,

which means that they were probably brought to America

by Roanoke settlers.

[narrator] The excavations at Site X also revealed another

essential artifact.

Archeologists found a small copper tube

that would have been used to secure wool fibers.

These all amount to small, but very significant finds.

This piece of metal is what's called an aglet.

It's kind of like what you would have

at the end of your shoelace.

This little thing is important because it went out of fashion

in the first half of the 17th century,

meaning that it could definitely have belonged

to a Roanoke colonist.

- What appears to have happened is that something

caused the settlers at Roanoke to disperse across the region.

It's impossible to say what the trigger was,

but life definitely wouldn't have been easy on that island.

- Life would have been easier if and when they integrated

with those indigenous communities that accepted them.

But because no single one would have been able

to absorb dozens of people at once,

the English of Roanoke would have had

to join various communities, those at Site X and

on Hatteras Island, likely being two of them.

And as for that patch on John White's map

that revealed the possible location

of the Roanoke settlers, why was that made secret

in the first place?

While the map was drawn by White,

it was done at the behest of his employer, Sir Walter Raleigh.

And remember, White also had his daughter

and grandchild there, so he may have had some

personal stakes as well.

[narrator] Today, the National Park Service Fort Raleigh Historic Site

has an exhibition dedicated to the vanished colony,

with the exception of the known few at Site X

and perhaps Hatteras, what's become of all the others?

A conclusive answer is unsure.

The settlers of Roanoke seem to have vanished into history.

♪ ♪

In the heart of the wild and unforgiving North Atlantic,

just a few miles off the eastern coast of Canada,

lies Sable Island, a 26-mile-long strip

of sand and grass.

It's called the Graveyard of the North Atlantic,

as 350 ships have been dashed against the island's

ragged coasts.

With that kind of history, it's no surprise that

there are boatloads of ghost stories

about this place,

including one about a shipwrecked woman

who wanders the beaches.

Legend has it that thieves cut off her finger

to steal a valuable ring, and that she's spending eternity

searching for both the ring and the thieves.

- Despite how grim and eerie that all sounds,

the island is also quite beautiful.

Even the name is pretty, Sable.

Derived from the French 'sable', it means sand.

- There's certainly a lot of sand here,

and not much else.

There's actually only a single tree on the island.

You can't even visit it without a permit.

Most of the people on the mainland

have not set foot here, and likely never will.

A 90-minute charter flight to the island

costs $1,500 US, and visitors aren't even

allowed to stay overnight.

[narrator] Sable Island's remoteness makes a bizarre

discovery here in 1993 even stranger.

A local researcher who spends a good part of the year

on the island is out on the beach doing her usual patrol

when she crosses paths with a seal.

Actually, a few of them.

[narrator] Between December and February, up to 400,000 gray

seals come to the island to mate and give birth.

It's the largest gray seal breeding colony in the world.

Okay, finding a seal, or even several seals,

on the coast of a Nova Scotian island

is like finding hay in a barn.

This is the North Atlantic. This is their playground.

But these seals aren't playing.

They're dead.

Now, around here, one dead seal, even 10 dead seals,

is not exactly a case for the CSI.

The ocean's a brutal place.

But what's startling here is how these seals died.

They have been sliced open.

The bodies have peculiar corkscrew cuts

that start at the head and spiral around the body.

The wound goes right through their skin and blubber.

In some cases, a chunk of the skin and blubber is gone,

but the tissue and muscle remain intact.

[narrator] By 1996, 400 such carcasses had washed up

on the windswept beaches of Sable Island.

And by 2001, the number is over 4,000.

And it's not just one kind of seal.

It's five.

They find gray, harp, harbor, hooded, and ringed seals.

What could be ravaging these poor creatures?

[Anthea Nardi] At first glance, this looks like the handiwork

of humans, as the wounds appear intentional and precise.

[Anthony Cantor] But if you follow that logic,

there are a few problems.

First, how?

This is an extremely remote place

with seriously restricted access.

For someone to regularly come to the island,

unnoticed, and kill thousands of seals,

that seems very unlikely.

The second thing to ask is, why?

What would be the point of killing these seals

and not taking their meat or their skin?

No, they were probably not killed by humans.

[narrator] Some believe these seals were attacked

by natural predators.

That would be a reasonable theory in some locations,

but what's on this island to hurt them?

There's nothing except for some birds and horses,

lots and lots of them.

If you fly over the island, you'll see something truly wild.

More than 500 horses running free.

It's just breathtaking.

[narrator] It was once thought the horses were from

the many shipwrecks around the island.

But they were likely placed here for breeding

after being taken from the French settlers

in the mid-1700s.

Regardless of how the horses ended up here,

could a horse be a predator?

They're herbivores.

There's no way they did it.

So what did?

- So if it's not a natural phenomenon

or the handiwork of humankind, could it be something

in the middle, like a boat or

specifically a propeller?

Generally, a ship's propeller spins

and has a smooth edge that would likely leave the seal

with a simple cut before pushing it away.

It would need to be held in place for a spiral cut.

Beyond those mechanics, if you look at the

wounds from propellers on animals like manatees,

they look nothing like that.

So this is likely not due to propellers.

[narrator] Another clue surfaces.

As the amount of bodies rises, a small number of

seals are found with crescent-shaped bite marks.

The North Atlantic is home to great white sharks.

So maybe they are making a meal of the seals.

Although there are lots of great white sharks

in this region in the warmer months,

the dead seals were generally appearing

in January and February, when the great whites

had already sailed off to warmer waters.

So it's unlikely that great white sharks

are the mass murderers here.

So what other shark species might be responsible?

Black dogfish?

Too small, and the seals would have made short work of them.

Orca? The tooth pattern just doesn't fit.

The Bluntnose Sixgill shark,

the Blue Shark, and the Tiger Shark

have only been found in this area as juveniles,

and juveniles would be too small to inflict

that kind of damage on an adult seal.

[Anthony Cantor] Even though some have dismissed the idea

that these wounds are the work of marine predators,

there is one more candidate, the Greenland shark.

[narrator] The Greenland shark can reach up

to a staggering 23 feet long and weigh over 2,000 pounds.

They're not just huge. They have incredible longevity.

Some are 500 years old.

Think about it, that's twice as old as the United States.

[James Ellis] While their sluggish nature doesn't make

them the most likely candidates, there's still some damning

evidence against them, specifically their teeth.

The upper and lower teeth of the Greenland shark

are very different.

The upper teeth are pointed and sharp for clamping.

The lower are more blade-like.

[Anthony Cantor] And the corkscrew tearing might be the

result of the seal's natural impulse to escape by twisting,

a reflex which tragically compounds the damage.

[narrator] But as experts settle on their verdict,

similar corpses begin appearing on the other side of the ocean,

on the coasts of England, Scotland, and Ireland,

and what they find calls the Greenland shark theory

into question.

And a bunch of the seal carcasses in the UK

are found in July, when the temperatures are way warmer

than the Greenland sharks prefer.

The propeller theory had been initially discarded

because it seemed propellers would just totally gash

the seal rather than produce the corkscrew pattern.

But what if the culprit was a specific kind of propeller?

[narrator] Ducted propellers are propellers

encased in short tubes that could possibly

keep the seal in place, allowing for the

corkscrew tearing pattern.

It's the kind of propeller often seen in tugboats

that service oil rigs or offshore wind farms.

When UK researchers cross-referenced

the discovery dates of the corkscrew-injured seals

with local shipping records, they discover that

there were vessels with ducted propellers nearby.

[narrator] To further test their theory, researchers make small

wax models of seals and put them into

model duct propellers.

The result, the same corkscrew injuries

that were found on the seals in the UK and on Sable Island.

Pretty convincing stuff, but many of the seals

found dead on Sable Island were still warm.

So it was most likely they were killed close to shore,

not by an invisible or distant boat.

So with the propeller dismissed,

the Greenland sharks kind of are leading suspect.

But before we throw the book at them,

let's just explore one more possibility.

What if, plot twist, the killers are the seals themselves?

Some people call seals 'sea doggos'

due to their whiskers and general cuteness,

but I wouldn't be quick to cuddle one anytime soon.

Look at their mouths.

They have the fangs and teeth of a dangerous predator.

[narrator] The seal's predatory power is very clear

when a shocking photo bomb blows the case wide open.

A marine biologist is documenting a

seal breeding colony on the Isle of May in the UK

when an adult male seal appears in the background.

He attacks a seal pup, his claws causing that

distinctive corkscrew cut.

He then basically pushes his jaw into the wound

and manages to swallow some of the pup's blubber and skin.

This is not a one-time thing.

Once the video is seen, the seal is tracked

and he's seen doing it again and again.

He's not the only one.

This is not normal behavior for seals

and it's unclear why it's happening,

but whatever the reason, British experts

are now confident that these bizarre seal deaths

are the result of cannibalism.

[narrator] While the case of these killings is

closed for most, others are unconvinced.

For them, the murder mystery of Sable Island remains unsolved.