Original subtitles

STEVE BACKSHALL: Deep in the ocean's dark abyss,

20,000ft below the surface,

in the freezing depths,

lurks a strange, unexplored world...

..where alienlike creatures...

..grow to terrifying sizes

and are the ruthless predators...

of the abyss.

Many look like something from our darkest imaginations...

..while some create the myths and legends of the deep.

Weird creatures with alien ways to hunt... and kill.

It's said we know more about Mars than we do our deep seas

and the strange life forms within it.

This series will uncover the great unknown,

come face to face with some of the sea's most elusive creatures...

..and use cutting-edge technology...

..to reveal behaviour that has never been captured.

These are the unknown and unseen monsters of the deep.

In this programme, we meet the giants.

From the massive monsters living in the deepest oceans

to the strange creatures that live longer than any other animal.

It's a giant isopod.

That is extraordinary.

From the most voracious super-predators...

..to the enigmatic giants that haunt our subconscious.

How did you feel when you saw that for the first time?

It was such an incredible "It's happening" moment.

Today, lurking in the dark abyss,

at depths of 1,000m or more,

are giants that are rarely seen.

Their presence revealed only through tantalising glimpses.

One, more than any, has gripped the dark recesses of our imaginations.

One of the most fascinating things about our seas

is that so many of its monsters

remained hidden for hundreds of years.

Tales of the giant squid go back centuries,

and usually from first-hand narratives

of sailors who encountered them while they were far out at sea.

And these gave rise to legends of mythical beasts like the kraken,

a creature that could engulf ships and drag them down to their doom.

In the mid-1800s, a ship's captain even described

three men being seized off the coast of Africa.

But hard evidence of these enigmatic monsters was difficult to come by.

Then, in 1873, a Newfoundland fisherman, Theophilus Picot,

had an actual encounter with a giant squid.

He spotted something floating at the surface,

and as he approached, realised that it seemed to be alive.

So he prodded it with his boat hook,

and, as he writes, that aroused the creature's fury.

And so it engulfed his boat with its tentacles and arms

and started biting with its beak.

And so he responded by hitting it with an axe.

This severed one of the feeding tentacles,

which was 19ft in length,

as long as a giraffe is tall.

Finally, it had been established without doubt

that the giant squid was real.

So what is the truth behind the legend?

It was only in 2004 that a complete giant squid was finally recovered.

It resides in a tank deep below the Natural History Museum.

Up to 13m in length, weighing more than half a tonne,

this is Archie, the female giant squid...

..deriving from the scientific name Architeuthis.

The person responsible for preserving Archie

is curator Jon Ablett.

So, to get a complete, intact specimen is incredibly rare.

I was contacted back in 2004

by a fisheries agency in the Falkland Islands,

and some fishermen pulled up their nets

and they found this in it.

And they caught her at a depth of about 200m,

which is probably near the kind of limit

to where giant squid occur towards the surface of the water.

We froze it on the Falklands and shipped it over

all the way back to the UK.

Then I defrosted it. It took about four days to defrost.

Because giant squid contain ammonia,

I basically made the whole building smell of urine.

So, now she is stored in the basement, in the tank room,

of the Darwin Centre at the Natural History Museum in London,

and the tank is actually manufactured

by the same people that make the tanks for Damien Hirst's artwork.

So I think she's my own piece of art.

Finally, we are able to examine the elusive giant.

So, at the end of Archie, at the tentacle,

it's an expanded area at the end called the tentacle club,

and this is where the largest suckers are found.

These suckers are about 2cm, 2.5cm, so really quite large.

And if you see, on the edge of each of the suckers,

it has this sawtooth firm edge,

which really helps it to dig in and grab its prey

when it shoots out its tentacles.

And if you move further back, you can see,

not only does it have the two long tentacles,

but back here, it has the shorter eight arms.

And these arms are much more muscular.

And there's quite a big difference between them,

because on the arms, you find suckers

along the whole length of the appendage,

whereas on the tentacles,

you only find these suckers right at the very end.

This is the head,

and the beak is actually at the top of the arms here.

Then behind it, you've got the eye,

and the brain is situated sort of just below the eye,

and it's doughnut-shaped to let the food pass through.

This is a beak from a giant squid,

and they're made of chitin,

a very strong substance, and incredibly sharp at the tip,

and allows to cut through bone, flesh, shell, whatever it's eating.

So, when a giant squid feeds on its prey,

grabs it with its large, fast-moving tentacles,

pulls it in to the much stronger arms where it can hold it,

and then you basically get nibbled to death.

It's kind of a death by a thousand nibbles,

macerating the food before it swallows it.

Because Archie is so precious,

to see the inner workings of the giant squid,

we need to examine a less complete specimen.

This is actually another specimen of giant squid.

This one came from Canada.

So, this area here is the mantle,

and this is where all the organs are stored.

So kind of everything that would be in our body.

We've got here one of the gills.

They actually have two large gills to extract the oxygen.

And they actually have three hearts.

So they have a central systematic heart,

and each gill has its own little heart.

So this whole mantle area fills up with water.

So it sucks the water in through here

and then pushed at great pressure through this funnel

to allow it to move very quickly, just like a jet ski.

So, giant squid normally live deep in the ocean.

So we're talking maybe over 1,000m,

and we don't think they can actually survive above 150m.

And what about the giant squid at the surface,

encountered by Theophilus Picot in 1873?

It seems likely that the "attack"

was simply the squid in its death throes,

as they only come to the surface when they're dying.

Archie, no matter how large and impressive,

is still a dead squid and can only tell us so much.

In life, is it an aggressive hunter-predator

or a passive opportunist?

No-one had seen the animal alive...

..because, unlike 90% of life in the sea

that lives in the upper two layers, in the sunlight and twilight zones,

the giant squid usually lives at great depth,

nearly a kilometre down,

near a layer called the midnight zone,

where light can no longer penetrate.

'It was while exploring these depths

'that marine biologist Nathan Robinson

'had only the second-ever squid encounter of the giant kind.'

I'm meeting him in the workshop

of the UK's home of deep-water exploration

at the National Oceanography Centre.

So, talk us through how you actually captured these images.

OK, so, this is a visual lure. This is our e-Jelly.

When we're trying to attract an animal like a giant squid,

you've got to think like a giant squid.

A giant squid has the largest eye in the animal kingdom.

They have an eye that can be around 12 inches, like the size of my head.

A giant squid's world is visual.

So, there's a deep-sea jellyfish

that creates a blue pinwheel bioluminescent display,

and it does it to attract other big animals

to eat the animal that's eating it.

So we create this biomimicked display,

and then that seems to be the perfect lure

to bring in a giant squid.

So, the imagery is very fleeting,

but you see this sinuous serpentine shape coming out of the darkness.

So, at this point, we'd reviewed about 120 hours of footage,

and so much of this is nothingness.

And then finally, this object came in

and then just unfolded its tentacles, attacked the bait.

It was such an incredible, like, "It's happening" moment.

And how deep down is this? This is at 750m.

It's surprisingly tentative, isn't it?

It's tasting.

It's figuring out, "Is this something I can eat?"

And the second it realises that it isn't what it thinks it is,

it just says, "No, I wanna go somewhere else."

What do you think this tells us

about how the giant squid is likely to hunt

and the kind of things that it's likely to have as its main prey?

So, there's big debate for a long time

as to whether giant squid are active predators.

This footage shows that it can actively pursue prey.

We have that five minutes

of it slowly just trailing behind our bait.

If you're actively hunting down your prey,

you're gonna need to eat more.

I've been underwater with Humboldt squid, big ones,

and one of the first things that they very obviously target

is other Humboldt squid. Yes.

Do you think that that is likely with the giant squid?

Could they be cannibalistic? I definitely think they could.

And when you're a deep-sea animal like the giant squids,

anything you find that's a potential meal, you're gonna eat.

So if they're encountering smaller and younger giant squid out there,

they could see that as their next big meal.

That's life in the deep sea.

A giant squid that washed up recently in Spain

showed signs of a lethal struggle.

Its injuries revealed it had been attacked

and mortally wounded by another giant squid.

It would appear that giant squid are not only aggressive hunters,

they are cannibals.

It's astonishing to think that this huge giant...

..despite being known about for centuries,

has only ever been filmed twice in the wild.

The biggest animal alive today, the blue whale...

..can be more than 30m in length and exceed 200 tonnes in weight.

And yet, encounters with them are rare.

And this raises an interesting question -

could our seas be hiding even greater mysteries?

And could the seas of the past hold even bigger monsters?

STEVE BACKSHALL: The biggest creatures on the planet

are found in our seas.

Some, hidden in the dark abyss, like the elusive giant squid.

Others, close to the surface, like the great whales.

The world's biggest animal is the blue whale.

But how and why do they get so big?

Baleen whales feed on prey that is the tiniest fraction of their size.

And in the seas of the past,

there were monsters that were almost as big...

..but far, far more ferocious.

One strategy to allow an animal to grow big

is to be able to take on big prey.

And to do that, it really helps if you have fearsome teeth.

That way, you get more bang for your bite.

So, this is the tooth of a great white shark.

It is scalpel-sharp, serrated at the edges.

Found throughout the oceans of today,

the biggest have been over six metres in length

and weighed as much as two tonnes.

With a mouthful of 300 teeth,

the great white shark is a formidable killer.

But this is the tooth of Otodus.

It is one of its relatives. It's extinct.

And I guess we know it better as the megalodon.

Megalodon lived in seas not so different from today

until around 2.5 million years ago.

For the most part, all we have of them is their teeth,

which grew up to 18cm in length.

The ancestors of our own great white shark were around at the same time.

But megalodon was only a distant cousin.

A much, much bigger cousin.

This tooth is a similar profile in shape but far, far bigger.

And there is no doubt that this had one of the biggest bites

on the planet.

Megalodon shares the sea with eight-metre whales.

The question is: how big is the prey that a megalodon can tackle?

At London's Natural History Museum,

Curator of Palaeobiology Emma Bernard has evidence

that could provide an answer.

This is a piece of fossilised whale bone

that's five-and-a-half million years old.

And this would belong to a whale about seven,

maybe eight metres in length.

So quite a big animal.

It's got these strange markings all the way across the bone.

They're actually bite marks.

And because of the depth and the distance between them,

it would have had to be a very large predator.

And that predator would have been megalodon.

The teeth of megalodon are the stuff of legend,

and the museum has over 300 of them collected from all over the world.

So, the name megalodon actually means "big tooth".

And I think that's quite clear to see with this one here.

And this would have been from a fully grown large megalodon.

It's also got this serrated edge all the way down the tooth.

And this serrated edge is very similar to a knife.

So, a megalodon tooth is really like a dagger

and a steak knife all rolled into one,

into an amazing, effective killing tool.

So, as it's biting down on its prey,

these teeth would often come in contact with the bones.

Like all sharks,

megalodon's skeletons are made of cartilage rather than bone,

so fossils are extremely rare.

Previously, megalodon's size estimates

were done by simply upscaling a great white shark

and were believed to be around 14m in length.

A megalodon backbone was discovered in Belgium in 1843.

But when Jack Cooper from Swansea University

re-examined it in 2022,

he made an incredible discovery...

..that changes everything we thought we knew about megalodon.

Jack Cooper is more familiar with the meg than just about anyone,

including Jason Statham.

He's reconstructed the monster using its backbone

and discovered something about the meg

that makes it even more terrifying.

You know, a shark can replace

tens of thousands of teeth during its lifetime,

and those are so well represented in the fossil record.

But to find a vertebrae, I mean, that's really special.

Even one vertebrae is incredibly rare.

And this spinal column from Belgium

is the only one of its kind in the world.

But when we reconstructed the spine,

we found that that alone was over 11m long,

and that brought the whole total length closer to 16m.

But even that is not the biggest sizes we know of a megalodon.

We have estimated that megalodon could get as big as 20m.

That is gigantic.

Well, should we get an idea of that size?

So, five metres.

I've swum alongside great whites at that kind of size,

but six metres.

So around about there. Let's keep going.

If we get a bit bigger still, we're going on

and up to our biggest fish species in the world today.

12m is a pretty good size for a whale shark.

But that is not a fierce predator like a megalodon.

It's, you know, feeding on plankton and small fish.

By comparison, megalodon was eating

almost 100,000 kilocalories every single day,

and that is about 20 times the amount of food

that a great white needs.

Megalodon was consuming whales up to seven or eight metres long.

That is insane.

But we're not there yet. They get bigger still, right?

16.

So, 16.

Length of the shark measured by its vertebrae.

That is the size of our model shark.

Already about three times bigger than our great white sharks.

But we still have some growing to do for our megalodon.

17, 18.

And, unbelievably...

..is there, 20m.

That's vast.

You could fit almost ten people in line,

and that would still have room to spare for megalodon.

So, what, then, were the main benefits of this enormous size?

Well, one of the key benefits of being so big

is that you have less competition by eating such big prey.

So that allows megalodon to eat pretty much whatever it wants

and not have to worry about that many competitors.

A 20-metre megalodon, how big is its mouth gonna be?

That mouth is going to be well over 1.8m tall,

so it can easily swallow a person.

You and me could both stand in a shark's mouth at that size.

'Jack's work on megalodon gives us the best picture ever

'of this super predator.'

Unlike most other fish, this is warm-blooded.

They can cruise faster than any other shark,

able to cover vast distances in search of prey.

It was a whale killer.

Megalodon could dispatch an eight-metre whale

in five or six bites.

Giant predators like megalodon are truly awesome.

They are dominant super killers.

And yet megalodon went extinct.

It turns out giants have an inherent weakness.

So long as prey is abundant,

being a giant apex predator means you rule the seas.

But if prey becomes scarce...

..all of a sudden, gigantism can be a big problem.

All it took was a slight change in the climate...

..which made their main prey harder to find.

Very quickly, one of the most gigantic predators

the world had ever seen disappeared.

But you can't keep gigantism down for long.

The loss of megalodon left a vacancy.

An opportunity for another animal to become a giant.

It was only after the extinction of megalodon

that baleen whales evolved into today's generation of giants.

Today's baleen whales can grow even bigger.

Because even though they feed on small prey,

by lunge feeding,

they can eat huge amounts

in what is effectively a single bite...

..and so can grow to enormous sizes.

The only limit is the availability and concentration of their food.

They're so big, they're virtually invulnerable.

But there is one monster that rises from the deep

to attack the giants of today.

From sperm whales...

..to orca and great white sharks,

alarming injuries have been seen on giants.

But just what is responsible for these bizarre wounds?

STEVE BACKSHALL: Becoming a giant might be difficult,

but the rewards justify the efforts,

making the animal pretty much invulnerable.

But given their vast size,

is there anything hidden in the depths of our seas

that poses a threat to today's ocean giants?

Well, there might be.

Strange injuries on some of our ocean's biggest animals

suggest a mysterious assassin lurking in the depths.

Even humans have been attacked.

What kind of monster is responsible for these vicious attacks?

Marine researcher Jethro Reading

at the National Oceanography Centre could have the answer.

These wounds are bizarre because they don't look natural.

They look surgical.

We find these injuries on large predators.

Here, we've got an injury on the side of a sperm whale.

Similarly, we've got injuries on orcas...

..great white sharks.

And something is attacking them.

Among the NOC's deep-water specimen collection,

Jethro suspects he knows the culprit.

This is the bizarre cookiecutter shark.

Here she is.

She's got this, like, little smiley mouth.

That mouth is actually concealing quite a sort of nasty little secret.

It's only half a metre long but packs a fearsome bite.

The cookiecutter shark has the largest teeth

for its body size of any shark that we know of.

This whole section would sort of hinge down

to create this circular mouth. And on the lower jaw,

these huge teeth are fused into this single saw-like structures.

The cookiecutter shark spends most of its time deep in tropical oceans,

up to 3,500m down in the midnight zone.

But to feed, every dusk,

it rises three kilometres to the sunlight zone.

It's rarely seen and has never been filmed feeding.

Having impressive jaws is only part of the cookiecutter's secret.

While it may look sort of rather drab and brown here,

this animal glows.

It shines a bright blue light out of its underside.

These dark spots all over the body are photophores.

That's organs in the skin that produce light.

They are bioluminescent,

and they're missing from this collar around the neck.

It uses this bioluminescence to hunt,

making its profile smaller,

trying to lure in potential victims.

Here, we've got a little setup that gives you an idea

of what you might be seeing if you are a predator in the deep sea.

So, it has a very, very strong silhouette

against the sunlight from above.

But with its glowing disguise, it looks much smaller.

This dark colour really stands out,

and it's thought that this dark colour

is going to resemble a small fish.

By luring their victim close,

the cookiecutter gets its prey just where it wants them.

And it may be that when these predators

investigate the cookiecutter,

lured in by this false fish silhouette,

the cookiecutter turns it around on them

and takes a chunk out of them instead.

And the hunted very quickly becomes the hunter.

Deploying its razor saw-like teeth...

..the shark latches onto its prey

and creates a suction seal with its lips.

It pivots around in a circle,

using the saw-like lower teeth

to cut a near perfect plug of muscle from its victim.

While the cookiecutter shark takes advantage of giants at the surface,

to find the biggest giants of all,

we need to descend into deeper waters.

Because it's here that we find abyssal giants

or the giants of the deep.

The Greenland shark is found in the North Atlantic

up to 2,000m deep...

..with temperatures around freezing in total darkness.

Here, food is very hard to come by.

And yet the Greenland shark grows to seven metres in length

and can live up to 500 years.

Some of the sharks living today

could have been around while Shakespeare was writing his plays.

They are the longest-living vertebrates on Earth.

They do so by taking things easy. They slow their metabolism.

They take their time over everything.

It takes 150 years to become an adult.

Greenland sharks are deep-sea giants.

But the most extreme examples of so-called abyssal gigantism

are not sharks.

And they're not squid either.

Tammy Horton at the National Oceanography Centre

is no stranger to deep-sea giants.

But first, she has a miniature to show me.

This is a shallow-water sea spider. And they're called sea spiders...

People might think they're quite closely related to spiders,

but they're actually quite distant.

It doesn't actually have much of a body.

The trunk is pretty small,

and the majority of the animal is legs.

This is a female specimen, and you can see its eggs,

which are held inside its legs all the way down there.

Oh, are they really? STEVE GASPS

That's amazing. So they're absolutely amazing.

What's this here? That's an egg mass?

Yes. We can zoom in a little bit to show you that.

Because there's so little space in their trunk,

their digestive systems, everything, their reproductive systems

stretches down into their legs.

This one here is about the size of my little fingernail.

Yes, but they can get a hell of a lot bigger.

To find the giants, you need to go deep.

That one's heavy, too. Oh, you're not lying.

After you. Thanks.

So, these were collected at 2,600m?

Yes, at the Mid-Atlantic Ridge.

And these are... STEVE GASPS

No. ..giant sea spiders.

That's insane! Yeah.

So, the deep-sea version

of that teeny-tiny fingernail-size sea spider is a giant.

That's right. These are true giants.

And I think the largest specimen of this ever recorded

had a 70-centimetre leg span.

70cm? 70cm.

Can I take one? Yes, please do.

Wow.

So, it's the same animal, but a whole different scale.

There are instances where deep-sea or abyssal giants

are 700 times bigger than their shallow-water equivalents.

And some look like they're from another world.

So, this is a giant isopod.

That is extraordinary.

What an alien.

This is the largest isopod known on this planet.

It looks like a super-sized woodlouse.

That's right.

I would imagine to many people, that would be their worst nightmare.

That is absolutely extraordinary.

It's grotesque but absolutely fascinating.

It's amazing, isn't it?

So, do you have a shallow-sea one for size comparison? Yes.

So, that's the same creature, just on a whole different scale.

This is effectively a shallow-water version of this giant isopod.

This is only found in deep sea.

'Proportionally, these deep-sea monsters

'are the biggest giants of all.'

This whole idea of deep-sea or abyssal gigantism,

the capacity to get really, really big,

are there any ideas of why that might happen?

One of the things to remember about living in the deep sea

is that food is really scarce,

and that means that these animals

have to really wait out long periods of time in between meals,

and then they have to travel longer distances to find their food.

Being larger, then, would give them a competitive advantage.

It's quite counterintuitive, isn't it?

It is quite counterintuitive that there are some animals

that are really, really big in this food-poor environment.

So they live a very slowed down sort of lifestyle.

Their metabolic rate is slower.

So, living slow, living long,

these can be a real advantage. Absolutely.

Animals with a low metabolism that live life in the slow lane

can make the best of meagre resources in their environment

and grow to become giants,

while those at the surface, like the great whales,

become giants by feeding efficiently.

The sea continually produces giants.

And a recent discovery might be about to dethrone the blue whale

as the largest giant that has ever lived on our planet.

STEVE BACKSHALL: The biggest giant on our planet is the blue whale.

Averaging 24m long and weighing 180 tonnes...

..it was thought that no animal had ever grown bigger.

But hiding in rocks laid in ancient seas are clues

that could change all that.

Despite their size, today's giants of the sea

can be surprisingly elusive and difficult to find.

It raises a fascinating question.

Just how big do these sea monsters get?

Some recent clues have given us the idea

that the biggest giants of all time lived in the seas of the past.

That monsters existed in the dim and distant past

has been known about for a long time.

In fact, the first discoveries of ancient marine reptiles

was right here on this stretch of coastline, the Jurassic Coast.

This was one of those monsters, as was this.

These exquisite ichthyosaurs are at the Etches Collection in Dorset.

The thing that strikes you is how similar they are

to a modern-day dolphin.

So, first of all, you've got this extended beak-like rostrum,

and inside there, rough, cone-shaped teeth

which would be perfect for catching slippery,

fast-moving prey like squid and fish.

And you can tell that's what it was eating

because its last meal is fossilised inside it.

The stomach contents are here,

and you can see little bits of fish bone.

So exactly the kind of thing that a modern-day dolphin would eat.

Ichthyosaurs, like dolphins, are air-breathing animals

and evolved from a land-living relative to live in the sea.

They were both shaped by evolution,

becoming streamlined for a life in the ocean.

It's a classic example of what biologists call

convergent evolution.

It has these four limbs here,

which are adapted into paddle-like flippers.

So those could be used to give it lift

and perhaps to move them in the water like rudders.

And this is truly remarkable.

On some ichthyosaur species,

they had the biggest eye socket of any known vertebrate.

So it's likely that those huge eyes drag in the light,

and they're used when they're hunting at depth in the darkness

for fast-moving prey.

From the largest-eyed species,

it's thought they could hunt by sight as deep as 500m.

Ichthyosaurs were one of the apex predators of their day.

And this one here was just a youngster.

They got to be much, much bigger than this.

Very recently, amateur fossil hunters

discovered a huge chunk of fossilised bone

on a beach in Somerset,

a fragment that hinted at something extraordinary.

Dr Dean Lomax is an expert on extinct marine reptiles.

He instantly recognised the significance of the discovery.

This is a giant jawbone from an ichthyosaur.

Now, it's just one bone, and this is... all together,

when we lay this out correctly, is over two metres long.

But it's incomplete.

We've got a much smaller skull

of an ichthyosaur that was probably about two metres long,

about the size of a modern-day dolphin.

And you can pick out this gigantic bone,

the very same bone in the bottom of the jaw here.

By comparing the fossils,

Dean can estimate the size of the giant's skull.

If you imagine, this is just one jawbone.

If you add all the other bits and pieces to this,

you can estimate that the skull might have been...

possibly up to five metres.

Truly a gigantic animal.

In 2024, Dean and his team named the new monster Ichthyotitan.

And when Dean looks at the jaw more closely,

there's another surprising discovery.

Not only can we learn a lot about bones from the outside,

but also on the inside.

So by taking this core sample,

you can see the structure of the bone inside here.

And all of these individual details

tell me that this animal was still growing,

and that potentially was probably something like a very late juvenile,

maybe, say, a teenager or an early subadult.

So an early adult.

So just how big is Ichthyotitan?

Based on the jawbone, Dean can estimate and plot out its size.

20, 21, 22.

We estimate that Ichthyotitan is about 25m long.

'That makes our Ichthyotitan longer than the average blue whale

'and a contender for one of the largest animals

'that ever lived.'

Because this individual was still growing when it died...

..it's possible that a fully grown adult Ichthyotitan

could reach 30m or more.

If Dean's calculations are correct,

he believes that Ichthyotitan could have been a real record breaker,

potentially able to challenge our very own blue whale

for the title of the largest animal ever known to have existed.

Just how do these monsters of the sea grow to be so immense?

One part of the puzzle is that seawater

will do the heavy lifting, quite literally.

Let's imagine that our water balloon here is a giant marine creature.

Well, its body will be supported by the water,

which is a denser medium than air.

It can hang here neutrally buoyant,

with no expenditure of energy whatsoever.

But take that same animal out of the water,

and it'll need a hefty so-called graviportal skeleton

just to stay upright.

Moving around requires an enormous amount of energy.

And at a critical mass, it'll simply collapse under its own weight.

As a result, it's only in the oceans

and in the deep, dark abyss where such huge animals can live.

One of the key advantages of reaching a huge size

is that you're pretty much off the menu for any other animal.

And in the sea, being bigger

means you can move through the water more efficiently.

From their eyes, we know ichthyosaurs were deep divers.

And a bigger body means a bigger oxygen-carrying capacity

and an ability to dive deeper for longer.

Today's deep-diving sperm whale can hold its breath for over 90 minutes.

For Ichthyotitan, getting big means not only is it a top predator,

but it can also hunt at depths

few other air-breathing animals can reach.

Today's giants, the great whales,

cover vast distances during their lives...

..and help perpetuate gigantism in the seas.

They circulate nutrients in the oceans they move through,

fertilising and boosting plankton production as they go.

Turbocharging the food chain,

ultimately enabling the giants at the top.

It's likely that all the giants we've seen

play a similar and critical role,

circulating and spreading nutrients through our oceans.

From megalodon to the modern whales of today.

Giants are not just inevitable in our oceans, they're essential.

Our oceans are the cradle of life on Earth.

Throughout history, they continually produce

the most formidable killers...

..the most immense giants.

And the more we explore,

the more of the ocean's weird and wonderful secrets we will discover.