Original subtitles

Narrator: Sharks and volcanoes.

Around the world, where you find one...

Man: Oh, (bleep)!

Narrator: ...You find the other.

Narrator: This isn't science fiction.

Man: Did we discover a sharkcano?

Yeah, we did.

Narrator: We're following world-renowned shark scientist

dr. Michael heithaus...

Mike heithaus: Keep the eyes open down there.

Narrator: ...As he dives

into this strange and dangerous realm...

Narrator: ...To uncover the powerful attraction...

Narrator: ...Between two fearsome forces of nature:

Volcanoes and sharks.

Narrator: An island on the far side of the world,

home to one of earth's most active volcanoes.

And some of the oceans' most deadly sharks.

This is the french island of la réunion,

a remote tropical paradise in the indian ocean.

But here, paradise is under siege by sharks.

Cécile lauret-stepler: A surfer was attacked here.

He was killed, he was bitten.

But I don't know exactly what happened, but yeah, he died.

He ended up dying.

Mike: More than other places in the world

where you wouldn't even normally think about shark attacks

when you go to the beach, in réunion, it's been a real issue.

Narrator: Dr. Michael heithaus is a marine ecologist

and a dean at florida international university.

He's been studying sharks for 25 years.

Mike: On réunion, there's no question

the government's taking this very seriously.

They've even instituted bans in terms of getting into the water.

Narrator: Swimming has been illegal on réunion since 2013.

It helps save lives, but some brave the breakers anyway.

The numbers tell the grisly tale.

Sharks have killed 11 people here since 2011.

13 others lost limbs but escaped with their lives.

That's 24 total shark attacks

on an island with fewer people than san francisco.

So what's going on here?

Nobody knows for sure.

Local experts believe that a perfect storm

of human and natural causes are to blame.

Mike agrees.

But he also sees something else.

A synergy between réunion's sharks

and its churning, active volcano.

Mike: When you look at réunion island,

it's not an island with a volcano.

It is a volcano that has built an island.

Narrator: This bleak landscape was laid in fire.

But it also reshapes the water.

Especially when it rains.

And in this tropical climate, it rains a lot.

Mike: That is called erosion.

And when you don't have plants to stabilize the soil,

you get sediment flowing downstream

and ultimately into the ocean.

And that's a recipe for turbid waters.

Narrator: So what does murky, turbid water

have to do with réunion's shark attacks?

Mike: In réunion, most of the attacks have been attributed

to one species:

Bull sharks.

Narrator: Bull sharks are distinguished

by their stout, cigar-shaped bodies,

broad, rounded snouts and small eyes.

They grow to about 7 feet long.

Mike: Yeah, bull sharks have quite a reputation

for being aggressive.

They get really big.

They've got a mouth toward the front of the head.

So they're built for taking big prey.

Most sharks will only attack prey

that's maybe 10% of their own size.

Bull sharks will attack prey that's almost their own size.

Fisherman: He's just shaking the hell out of me.

Man: Holy (bleep).

Mike: When most people think about sharks,

they think about white sharks.

You know, they've got those big eyes, they've got good vision,

a lot of times we see them in pretty clear water.

Bull sharks are a little bit different.

That eye is a lot smaller,

which might mean that they're less reliant on vision.

But what's beyond dispute

is that they have an amazing sense of smell.

So even if the water is really murky,

they should be able to find and catch prey fairly easily,

and, in fact, may have an advantage over prey

that don't have as good a sensory system.

Some other species, they like the clear water.

Bull sharks, they're perfectly at home

where it's really, really murky.

♪ ♪

we're off the west coast of réunion island,

and you can see the volcano disappearing up into the clouds

with these really steep sides, and you can see these ravines

that have been cut into the side of the volcano.

So when it rains heavily you get these huge torrents of water.

They come out the mouth of rivers like this,

and it'll basically make the ocean look like chocolate milk.

You know, the visibility will go to basically zero.

Narrator: Mike wants to observe the murky conditions

as a bull shark would: Underwater.

But swimming here is not only illegal,

it could also be lethal.

Mike: Since I'm not going to be getting in the water myself,

we're going to have to use the next best thing.

This is an underwater camera,

so think of a periscope, only going the other way.

And what this will do is let us get a view

of what's going on underwater,

see how good the visibility is, what's down there,

and especially what the water looks like

this close to the mouth of a river.

♪ ♪

ugh, yuck.

I am seeing a whole lot of greenish-blue gunk,

and you just see the light is just picking up

all of that particulate matter,

so the visibility here is not very good.

We're not seeing very far into the distance.

We're only seeing

maybe three or four feet away from the camera.

So those are the kind of waters that bull sharks

can still hunt in,

but prey would have a lot harder time getting away.

Narrator: About two thirds of the shark attacks

here on réunion have occurred

in turbid water conditions like these.

So just how well can bull sharks hunt

in murky, turbid water?

That's what mike wants to find out.

Since he can't do it safely in réunion,

he heads to the next best place:

Bimini island in the bahamas, famous for its bull sharks.

Mike's staging an experiment

to compare how well bull sharks can find food in clear water

versus turbid volcanic muck.

Mike: So, we're here in bimini

because it's got a really nice population of bull sharks.

Hopefully we're going to be able to entice a couple in

and we'll be able to see how quickly they can home in

on potential food sources,

when they're almost blinded from the turbidity.

That's gonna tell us a lot about how these sharks

might be hunting in other areas like réunion.

Narrator: He'll observe bull sharks feeding

in two different conditions:

First in clear visibility, and then in near-zero visibility.

Duncan brake: Hey, mike.

Mike: Hey. Ready?

Duncan: As ready as I think I can be.

Narrator: Mike's teaming up with duncan brake,

a veteran underwater cameraman

with years of experience swimming with sharks.

It'll be duncan's job to document

today's bull shark experiment.

Duncan: I've dove with sharks all around the world.

And the species that I'm probably the most wary of

when I'm in the water with is definitely the bull shark.

They can be very unpredictable.

I've seen their temperament snap in an instant.

Going from incredibly mellow, chilled out,

to just absolute chaos underwater.

Narrator: To raise the stakes even further,

today's experiment requires swimming with bull sharks

in near-blackout volcanic conditions.

Mike: Obviously there's no volcano here on bimini.

So, we're gonna stir up the bottom

and try to create zero visibility.

Narrator: Mike will use a blower to churn up sediment

and simulate turbid conditions,

like those found around réunion.

His job can be done safely inside a steel shark cage.

But for this experiment, duncan needs to capture

clear footage of the animals' behavior.

That means outside the cage.

A safety diver will watch his tail

and fight off any sharks that come too close.

Duncan: I was thinking maybe we'll just swap roles

and I'll go in the cage, and you can dive with the bull sharks.

Mike: I'm good.

(duncan laughs)

duncan: I don't usually mind diving with bull sharks.

But we're engineering a situation

where the visibility is going to suck,

and we're not going to have a clue

what the sharks are going to do.

So, yeah, I definitely grabbed the short straw jumping in.

Narrator: Once they stir up the bottom,

he'll basically be swimming blindfolded

with a bunch of bull sharks.

Mike: Yeah, I don't care how many dives you've done.

If you're in zero visibility,

especially with bull sharks, it's scary.

Narrator: First, the easy part: Feed the sharks in clear water.

Narrator: The bull sharks attack the food

the instant it hits the water.

♪ ♪

♪ ♪

♪ ♪

♪ ♪

narrator: In clear water,

the bull sharks grab the food almost instantly.

But mike needs to see how well they work in volcanic murk.

Narrator: Mike and duncan know

that once the blower kicks up the sandy bottom,

there's no going back.

Narrator: Dr. Michael heithaus and his partner,

cameraman duncan brake,

are swimming with a bunch of hungry bull sharks...

Narrator: ...And now they're doing it blindfolded.

Narrator: Mike and duncan are simulating volcanic conditions

to see how well bull sharks can locate food

in murky, turbid water.

It'll help answer mike's big, burning question:

Why are sharks attracted to volcanoes?

Narrator: An imaging sonar allows mike to see,

even in zero visibility.

Narrator: The bull sharks feed

in volcanic, zero-viz conditions

with absolutely no problem.

Narrator: Mike has all the data he needs.

They can pack it in.

(laughter)

narrator: Duncan's stingray scare

signals a good time to get out of the water.

Duncan: Whoa. Oh, man.

That really escalated quickly.

They were very calm and collected.

But then, when the visibility was reduced,

we completely lost the upper hand.

And it was definitely their game.

Sorry.

(laughs)

narrator: For mike, it was a different ballgame.

The imaging sonar allowed him

to see the bull shark behavior perfectly.

Mike: That was amazing.

What incredible animals.

And then to be able to stir up the bottom

and use the sonar to see how well they navigate

that turbid water, absolutely amazing.

Narrator: Today's experiment has convinced mike

more than ever that bull sharks don't just tolerate

turbid, volcanic areas, they thrive in them.

Mike: In fact, the sharks may like those areas,

because it's like turning the lights out

for a lot of their prey.

That's kind of perfect hunting grounds for a bull shark.

Narrator: Bull sharks are intelligent predators.

If active volcanoes help create a perfect hunting ground

for them, it's easy to see why they hang out there.

But mike suspects there are even more reasons

sharks are drawn to volcanoes, both active and dormant.

To see if he's right,

he's heading back to the indian ocean

100 miles east of la réunion...

...To the volcanic island of mauritius.

Mike: Mauritius is about 8 million years old,

the volcano is dormant.

So what it really lets you do is kind of control conditions

for active volcano, dormant volcano.

And mauritius may give us a window

into what réunion might look like

several million years in the future.

Narrator: Unlike réunion,

mauritius's dormant volcano is covered in thick vegetation.

Mike wants to see if the marine environment

looks any different,

and how that impacts sharks and their behavior.

Once again, his underwater cameraman duncan brake

is along for the ride.

Duncan: So, yeah, we got out here

and it's looking a little bit sporty to me.

Pretty big seas after that storm rolled through.

Mike: Yeah! Yeah, and I've heard it's not the swells

on the surface that we need to be really careful of,

it's the currents underwater.

Duncan: What sharks are we expecting to see

in this volcanic topography?

Mike: Well, out here, probably grey reef sharks

are the most likely,

you know, they love being close to these drop-offs.

Narrator: Grey reef sharks are common

in this part of the world.

They grow up to 6 feet long,

and spend most of their lives around coastal areas and reefs.

Mike: Sometimes grey reef sharks can have a bit of an attitude.

So you really want to watch their behavior.

They will put their pectoral fins down and arch their back

and swim in a fairly exaggerated way

if they want you to get out of their space.

♪ ♪

♪ ♪

keep the eyes open down there.

Sharks, swell, see how this goes.

♪ ♪

♪ ♪

narrator: An undersea paradise.

Jagged volcanic terrain offers plenty of nooks and crannies

for corals to take hold

and for fish to hide from predators.

Narrator: The shark food is here, but where are the sharks?

Volcanic islands like mauritius have a shallow coastal ledge

with a sheer drop-off down to the ocean floor.

This is where reef sharks can typically be found.

But here, there's no sign,

and no protection from the open ocean currents.

(groaning)

♪ ♪

♪ ♪

narrator: These exposed conditions are dangerous.

But a volcanic formation looks to offer shelter.

Narrator: In here, mike and duncan

are safely out of the surge,

but they're not out of trouble.

Narrator: Off the coast of mauritius,

dr. Michael heithaus and cinematographer duncan brake

scramble to escape dangerous surge and ripping currents.

(muffled scream)

a narrow volcanic passage offers shelter...

And company.

Narrator: Today is their lucky day.

Narrator: They've been searching for sharks

all morning.

Mike wants to see how sharks benefit

from dormant volcanic islands, like mauritius.

They follow the shark and hope it leads them to more.

Narrator: Mike and duncan enter a large volcanic cavern

with crystal clear water.

Narrator: They keep their distance

and a sharp eye on the sharks.

Narrator: Mike is satisfied.

This healthy congregation of reef sharks is proof positive

that even dormant volcanoes have plenty to offer to sharks.

♪ ♪

♪ ♪

mike: Ah! That was fantastic.

Clear water, great volcanic island

and lots of sharks.

Woo!

Volcanic islands are these oases in the desert

where you've got food chains that are just thriving,

and that's going to attract animals from all over,

because it may be hundreds or thousands of miles

to the next closest land.

Narrator: The lush reefs of these remote volcanic islands

are a critical food source for sharks.

And not just the coastal species

that spend their entire lives there, like reef sharks,

but also for big, roaming, open-water monsters,

like the great white.

One such island is guadalupe,

a volcanic palisade in the pacific,

150 miles west of baja, mexico.

In 2010, cinematographer andy casagrande

was at guadalupe on assignment,

looking to film great white sharks.

He got a chilling, up-close-and-personal reminder

of how important these volcanic islands are

to big pelagic sharks.

Andy casagrande: The holy grail as a natural history cameraman

would be to capture great white sharks breeding, mating.

But the next best thing is feeding.

Narrator: Guadalupe's rugged shores

are an annual birthing ground

to many types of seals and sea lions,

like the enormous elephant seal.

Andy: Elephant seals can dive to like 8,000 feet,

hold their breath for 2 hours, and they're very intelligent.

So, they're prey species

that's not easy for a white shark to take down,

but very lucrative if they do.

Narrator: Elephant seals haul themselves out

onto guadalupe's rocky beaches twice a year,

to molt and mate.

Volcanic islands have steep underwater flanks,

allowing white sharks to come right up to the beach.

Mike: When they get in close to shore,

those white sharks know that they're going to be there.

And there's this narrow zone

where those seals have to get on and off islands,

where they're really at risk.

It's a very narrow window each year,

but it's a pretty important one.

Narrator: Andy is lucky enough to be there

at exactly the right time.

The sharks are lurking.

And the seals heading back to sea.

But andy's not seeing any action.

Andy: We're trying to capture these predation events.

We're striking out. We're not seeing anything.

Narrator: Then he notices a young elephant seal

making a break for deep water

and heading straight towards the white sharks.

Andy grabs his camera,

but in that split second, loses sight of the seal.

Andy: And then we capture these birds on the horizon.

Man: Here he comes.

Andy: And we see they're picking up bits of flesh.

We rush out there.

Really, there's lots of sea gulls and the splashing,

and just, you know, when I'm filming these events,

I almost get an out-of-body experience.

Narrator: Andy plunges his camera over the side.

He'll never forget what he sees next.

Andy: It is ferocious.

And the blood and the bubbles.

It literally is something out of a horror film.

Andy: It is ferocious.

It literally is something out of a horror film.

Narrator: In the pacific,

off the remote volcanic island of guadalupe,

cinematographer andy casagrande gets a close encounter

with raw nature.

A great white shark devouring an elephant seal.

Andy: There's a large elephant seal with its head missing.

The shark grabbed the carcass, thrashed it,

swam right up to us,

and basically, you know, gave us this,

"hey, come on in, the water's fine.

But I'm going to eat you along with this seal."

narrator: Andy opts to stay out of the water.

But big pelagic sharks like the great white

seem to understand that big, juicy meals,

like elephant seals,

can be scored around these remote volcanic outposts.

So the question is, if volcanic islands like guadalupe

are so tiny and isolated, how do sharks find them?

Dr. Michael heithaus believes the answer lies

with the shark's mysterious sixth sense.

Mike: If you think about the senses we have, we've got taste,

we've got touch, we've got sight,

we've got smell, we've got hearing.

Sharks have all of those.

And then they have a sense that we don't have.

They can detect electric fields,

and all living things give off those electric fields.

Narrator: This extra sense is thanks to a secret weapon:

The ampullae of lorenzini,

an array of pores spread across their snout

which picks up tiny changes in electric energy.

Mike: So this electrical sense that the sharks have

is incredible, it's almost like a super power.

Narrator: Sharks use their ampullae of lorenzini to hunt.

But scientists suspect sharks might use this same super sense

to navigate open ocean

and track down bountiful, remote volcanic islands.

Now mike is going to put it to the test.

He's teaming up with dr. Stephen kajiura,

a biologist and professor at florida atlantic university.

Steve's been studying the electrosensory capabilities

of sharks for years.

Stephen kajiura: All sharks have the ability to detect

electric fields in the environment

with their ampullae of lorenzini,

their electroreceptors.

And that's well-documented. They use that to find prey.

But it's interesting that electricity and magnetism

are closely intertwined.

Narrator: If sharks can detect magnetism

with their electrosensing superpower,

then they might also use it as a compass.

Mike: How?

It's probably got to do with being able to detect

weak magnetic fields, you know, the magnetic field of the earth.

But then if we think about volcanoes and that lava,

it's got lots of iron, which is naturally magnetic.

And so these lava flows in volcanic islands

could serve as waypoints for the sharks' sensory systems.

Narrator: But this theory all depends on one big question:

Can sharks detect magnetism?

♪ ♪

♪ ♪

mike: We're targeting hammerheads here,

and the reason for that

is that they've got lots of ampullae of lorenzini

so they should be fairly receptive to magnetic fields.

Of course, it doesn't hurt that they're also super cool.

Narrator: Hammerhead sharks are one of the ocean's

most distinctive predators.

Their broad head, called a cephalofoil,

acts like an airplane wing

and offers incredible maneuverability.

Large, powerful animals.

If provoked, they can get aggressive.

♪ ♪

stephen: Here we have a big 50-pound magnet.

This thing will lift an entire 50-pound steel block.

Mike: Yeah, those are strong.

I can't even come close to pulling them apart.

Narrator: Mike and steve's experiment

will use three magnets

to see if they trigger a reaction from the sharks.

Stephen: So, what we have here is not only the magnets,

but we also have the control treatments.

These are just aluminum blocks, also painted red.

They look identical, and so the shark won't be able to tell

by looking at them what's what,

but one's putting out a giant magnetic field

and one is, one is clearly not.

Mike: Okay.

So, we've got the control blocks and the experiment blocks.

They're the same size, they're the same shape,

they're the same color.

The only thing they differ in is one is magnetic and one is not.

Narrator: If steve's prediction is right,

the sharks will ignore the non-magnetic aluminum controls

and react to the magnets.

Duncan: Now, are you expecting the sharks,

are they gonna be going at the magnets

or are they gonna be, like, bounced off them?

What are you thinking you're gonna see down there?

Stephen: I don't know. It's hard to say.

It could go either way.

They could either be attracted to them, they could be repelled,

they could just ignore them completely.

Duncan: Alright, let's get suited up, get down there.

Mike: Put these in action.

♪ ♪

♪ ♪

narrator: It's an open-water laboratory.

The magnets in an arc to the right,

and the non-magnetic aluminum controls to the left.

Bait lures sharks over the test field.

Narrator: A slew of nurse sharks show up

with the hammerheads; a welcome bonus.

Narrator: Now mike and steve can look at how both species

react to the controls and magnetic blocks.

Narrator: In bimini, bahamas,

a team of experts are hoping

to entice a couple sharks for an experiment...

♪ ♪

...And get more than they bargained for.

♪ ♪

♪ ♪

narrator: They're testing if sharks

can detect magnetic fields.

The answer could unlock a major mystery

about how sharks find

far-flung, food-rich volcanic islands,

and whether the islands' naturally magnetic lava rock

is involved.

For this open-water experiment,

they arrange three powerful magnets to the right,

and three non-magnetic controls to the left.

Narrator: A big hammerhead glides over

the aluminum control without a glance

and heads for the magnets.

Narrator: Hammerheads ignore the aluminum,

clustering around the magnets.

The nurse sharks have a greater response.

Narrator: Nurse sharks are drawn to the magnets.

♪ ♪

♪ ♪

♪ ♪

♪ ♪

♪ ♪

♪ ♪

narrator: The sharks are agitated,

possibly by the magnets.

♪ ♪

♪ ♪

stephen: That was fantastic.

That was great.

So many hammerheads down there, all over the place.

Bunch of nurse sharks.

Nurse sharks were biting at the magnets.

Really neat dive all-round.

Mike: So that dive was crazy.

We had nine hammerheads there,

seemed like countless nurse sharks.

What we saw was that both the nurse sharks

and the hammerheads did respond to the magnets.

So, what we can take away from that

is that those weak magnetic fields

that are locked in volcanic rock

are something that the sharks could detect

and may be able to use in navigation.

♪ ♪

what scientists are starting to think

is that they're probably using the earth's magnetic field

to move around.

Now, when they get closer to volcanic islands,

they might be able to use that magnetic sense

to detect anomalies in the lava that formed when it cooled.

Narrator: Evidence suggests sharks use volcanic formations

to navigate, and volcanic islands as food pit stops

in the wide-open ocean.

But what about the volcanoes themselves?

Are sharks attracted to the actual churning heart

of these fiery furnaces?

♪ ♪

dr. Brennan phillips found the answer to that question,

almost by accident.

It's a case of truth being stranger than fiction.

Brennan phillips: Did we discover a sharkcano?

Yeah, we did.

Man: Oh, (bleep)!

(scream)

man: Holy (bleep).

Man: I'd be careful. Oh, (bleep)!

Man: Holy (bleep).

Brennan: What makes working around a volcano dangerous

is, well, it can erupt.

♪ ♪

so nothing over here just to show anything.

Narrator: National geographic grantee dr. Brennan phillips

studies underwater volcanoes.

He recently made an astounding discovery

that could blow the lid off of everything we know about sharks

and their attraction to volcanoes.

Brennan: My expedition to kavachi was about exploring

that volcano for the first time really

using real scientific equipment.

Narrator: Kavachi, an active underwater volcano

in the south pacific.

Its caldera, only 60 feet beneath the surface.

Brennan was there to analyze the volcano...

♪ ♪

...Not sea life.

Brennan: Kavachi is one of the most active submarine volcanoes

in the world, it can erupt at any time.

So probably the worst possible-case scenario

is that hot magma could land right into your boat

and it could burn a hole right through the hull of your boat

or even through you.

Narrator: Brennan's team launched a robotic camera

to get an up-close look.

Brennan: So it's a surface robot that they could drive

using a little remote control.

It had the camera on it and a few other sensors.

And the whole idea was it could be lost if it need be.

And we drove that right up into the volcano

as close as we possibly could

and waited, hoping that a big eruption would happen.

And we got it.

Man: Kind of getting-- oh, (bleep)!

Man: Oh, yeah!

Man: Holy (bleep).

Brennan: That eruption really caught us off guard.

It was very loud. It was very violent.

It's certainly shocking.

And everybody kind of snapped to and paid attention,

right when that went.

Man: Those guys gotta be careful.

Narrator: But brennan wasn't finished.

He wanted an even closer look.

Brennan: We actually wanted to get images

inside of the volcano, inside the crater of the volcano.

And so we used a national geographic drop camera,

which is an autonomous self-powered robot camera

that you can essentially chuck off the side of a boat,

it will sink, and for a set amount of time

that you program it for, it will record video.

Men: Three, two, one.

Narrator: The drop camera sank

straight into the roiling heart of the volcano.

Brennan: So we're watching the video,

and all we're seeing is a lot of cloudy water, orange water,

that we had come to expect inside that plume.

And then out of nowhere we saw a whole bunch of sharks

come right up to the camera,

and they're moving very, very quickly,

and darting back and forth.

When we saw that, we were, we were super shocked.

Man: Oh, nice. Look at that.

Narrator: Brennan captured something never seen before:

Live images of sharks swimming

inside the caldera of an active volcano.

Brennan: There's a lot of things going against them

going into that volcano,

between the heat and the caustic nature of all the water

that's coming out of the volcano.

Those are all bad things for fish and sharks to live in.

But these sharks seem to be fine and thriving in there.

My thought is that they do leave when there's an eruption,

that they can sense it and they get right out of the way.

This seems to be a community that is used to this activity.

Men: Whoa! Woo!

Mike: Dr. Phillips brought back evidence

that sharks were using this caldera.

And, you know, why are they there?

One reason might be there's some food,

another might be it's nice, warm water.

Sharks will move from warm to cold water

to get energy to feed, maybe digest optimally,

or even speed up reproduction.

Who doesn't like a hot tub?

Brennan: Did we discover a sharkcano?

Yeah, we did.

We found sharks living in a volcano.

I think it's highly likely that there are sharks

living in other submarine volcanoes.

Narrator: We may not know exactly why sharks are there,

but the incredible fact that sharks are inside a volcano

shows these two fearsome forces of nature

share a powerful connection.

Mike: Hey, sharks like volcanoes.

And if you think about it,

volcanoes are just really critical to ocean ecosystems.

I mean, they're these oases out in the middle of nowhere.

Without them, it would just be more open water.

There's no question,

we're really only scratching the surface of what we need to know

about how volcanoes affect sharks

and all the other species that call these waters home.

♪ ♪

captioned by side door media services