Original subtitles

Narrator: What's this sizzling sound

At the bottom of the ocean?

Nosal: What I can best describe as, like, a crackling sound,

Almost like when you put rice krispies into your milk.

Narrator: Why does this herd of wild elephants

Check into the same hotel every year?

Aryee: This clip is phenomenal.

It's incredible. It's amazing.

Narrator: And tourists in portland, maine,

Witness an astonishing sight.

A strange circle of bubbles suddenly appear.

What's causing that?

Narrator: Nature is awe-inspiring,

But sometimes it just doesn't make sense.

Man: I have never seen anything like this.

Our team of experts investigates the weirdest animal behavior...

That's amazing.

...And the most unexpected events...

What is causing that?

...Ever caught on camera.

My god!

These are...

August, 2016, western australia.

Above the waves, a peaceful,

Serene sea stretches as far as the eye can see.

But below the surface,

That serenity is shattered by a curious sound.

[ crackling ]

If you're out in the ocean scuba diving,

You can hear a lot of different things.

You can hear whales,

You can hear some fish, but there's one noise

That you might hear that's hard to explain.

What I can best describe as, like, a crackling sound,

Almost like when you put rice krispies into your milk.

And it gets louder and louder as you approach rocks.

Narrator: What could be causing

This bizarre underwater cacophony?

Nosal: If you're not familiar with the area you're diving in,

You might think that there's some sort of machine

Nearby creating this sound.

Maybe some sort of sonar equipment from a nearby boat

Or even a submarine.

Narrator: Sonar was developed during the first world war

To listen out for prowling german u-boats.

It works by pinging out sound waves,

Which bounce back off solid objects,

Indicating both how far away the object is and its size.

Nosal: There's two types of sonar -- active and passive.

Active sonar actually emits sound underwater at frequencies

That humans can even hear.

Narrator: Cargo ships, fishing vessels,

And military forces across the world all use it.

So, could sonar be the source of the mysterious sound?

Riskin: The problem with the sonar theory

Is that sonar tends to happen at regular intervals,

You know, with a rhythm.

Nosal: This is very different from the actual crackling you're hearing,

Which is kind of this constant chorus of white noise.

This doesn't seem like it's sonar.

Narrator: If the noise is not manmade,

Could it have a natural origin?

Nosal: We know that dolphins vocalize with each other.

They also use echolocation, where they will actually emit

Clicking sounds underwater.

Narrator: A dolphin's echolocation works in the same way as our sonar,

Helping them to navigate, find prey, or detect predators.

Dolphins can actually process hundreds and thousands

Of these clicks every single second.

And in an environment which is murky and dark,

This is really important.

So, could this sound be coming from dolphins?

Riskin: Problem with that theory is that sometimes,

This clicking noise happens where there are no dolphins,

So it's got to be something else.

Narrator: A military report from the second world war

May provide the answer.

Off the indonesian coast,

An american submarine crew heard the weird sound.

Fearing that the ominous crackle might be a japanese device

Designed to disrupt their sonar, they reported the incident.

After a year-long investigation and hundreds of acoustic tests,

Baffled scientists finally identify the culprit.

The solution is tiny.

It's this little nothing of an animal called the pistol shrimp.

Narrator: So, how does such a small creature

Make such a loud noise?

Meet billy the shrimp,

The fastest gun in the wet.

So, this pistol shrimp doesn't look so spectacular

Until you pay close attention to its claws.

Its claws are quite big and quite strong,

And there's a loading mechanism

And a release mechanism built into it.

Narrator: First, the claw is cocked.

One half is shaped like a plunger, the other a socket.

Nosal: But these parts are not coming together

Like a clap making a sound.

Those two parts come together with such force that water

Is displaced out of that area, creating a bubble.

Narrator: Blink and you'll miss it.

To see it properly, we have to slow it right down.

The bubble bullet swells

And then implodes almost instantaneously,

Generating a temperature nearly as hot

As the surface of the sun,

A flash of light, a powerful shock wave, and a loud bang.

Riskin: Pistol shrimp do this for a couple different reasons.

One reason is that they might want to stun prey.

If they do that "pah!" thing in front of their prey,

It kind of knocks them unconscious

So they can go grab them.

Nosal: But in addition to that,

It's thought to be used as a means of communication.

Narrator: The gun-toting crustaceans

Live in large colonies hidden in the sea bed.

That means what we can hear is the sound of thousands

All talking at the same time.

So you can imagine all the chatter

Amongst all of these shrimp in a reef.

That's what creates that constant crackling sound that,

Again, seems like white noise

When you spend enough time underwater.

Narrator: At the end of 1942,

When the u.S. Navy finally learned

What was causing the noise,

They found a clever way to use it to their advantage.

So, in world war ii, underwater,

Everyone was using sonar to see each other.

And so by hiding where there were pistol shrimp

Or playing the sounds of pistol shrimp,

Those american submarines

Could acoustically hide from the japanese boats.

That's really cool.

I like that a lot.

Okay.

♪

Narrator: November, 2017.

Mfuwe lodge in zambia.

The hotel is in busy season,

But not all the guests have got a reservation.

Aryee: This clip is phenomenal.

It's incredible. It's amazing.

You've got the world's largest land mammal,

The african elephant, walking through a hotel lobby.

That's a pretty amazing bit of footage.

There has to be a reason

Why the elephants are going through this lodge.

Midcap: They know exactly where they're going.

They're going to a mango tree.

Narrator: According to hotel staff,

This herd of wild elephants turns up every year

Just as the fruit becomes perfectly ripe.

The owners built the lodge around the mango tree,

Unaware that this was one

Of the elephants' favorite feeding grounds.

Oh, the baby -- he's just a few weeks old.

Hi, little one.

And this is at the end of the lunch.

This is the sugar coma.

Yes, we all experience that, don't we?

These african elephants finding this one mango tree

Is really impressive when you understand

That their home ranges are massive.

Up to 9,000 square kilometers.

That's the equivalent of around 180 manhattans

Or the size of connecticut.

Now, you try finding a mango tree in that.

Narrator: How do these elephants know when the mangos are ready to eat,

And how do they find their way back

Across such a vast area year after year?

One theory could be visual signs.

Could they be using, perhaps, landmarks

And specific geographical signs that are helping them build up

A picture of the environment to return back here?

Narrator: It's thought the elephants have been dropping by

For over 25 years,

Long before the hotel even existed.

So is it possible the herd are using features in the landscape

To find their way back?

Narrator: In zambia, wild elephants

Make an annual pilgrimage

To the mfuwe lodge to feast on juicy, ripe mangos.

But how do they find them in their vast home range?

One theory is they could be relying on visual cues,

Landmarks along the route to navigate by.

A lot of animals do rely on visual cues

To get around different locations and to find food.

Daly: Flying insects, such as wasps, recognize landmarks,

Such as stones or fallen leaves, before they leave their nest

So that they can navigate their way back.

And it's thought that mammals do this, too.

But remember that, in a dynamic, changing landscape,

So you rely on visual cues to try

And navigate your surroundings to try and find your resources.

So I think that they are relying on something else.

So, do they have some other mammoth skill

That they might be relying on?

Isn't there a famous saying about elephants?

Elephants never forget.

Narrator: Melissa schmitt has been working with elephants

For the last five years.

Today, she's meeting up with some old friends

In limpopo, south africa.

Schmitt: So, african elephants have an incredible memory.

It's quite remarkable.

They can actually remember life events,

Specific events in time.

They recognize individuals.

They could -- thank you --

They can recognize other herd members.

They also have a really remarkable memory

Of where resources are located in a landscape.

And memory's very advantageous,

Especially in an environment like an african savannah.

Narrator: And elephants definitely need to be experts

At finding sources of food.

Galante: African elephants need to consume

Up to 400 pounds of food a day.

So they'll spend up to 18 hours doing nothing

But looking for food.

Narrator: In the perilous african landscape,

Foraging for this incredible amount of food is no mean feat.

African savannahs are typically characterized by two seasons --

A wet season and a dry season.

During the dry season, the grass is brown.

Many of the deciduous trees have lost their leaves,

And water becomes much more scarce.

And having knowledge and memory of resources

That are longer lasting would be incredibly advantageous

For elephants to know

So they can revisit those resources.

Narrator: That means the bigger the area they patrol,

The better the chance at finding food.

And that's because even though they are roaming larger areas,

They're able to remember exactly where those resources are.

Whether it's a watering hole or a mango tree.

Narrator: Amazing.

It's an elephant's ability to mentally map

The location of vital resources

That guides them back here each year.

But there's another twist to this mango mystery.

Memory for elephants is a really important tool,

But in this case, memory can't explain

How the arrival of elephants

Coincides with the fruit ripening.

And that's because the time of year

When the fruit is ready to eat changes year on year

Depending on rainfall and climate.

Narrator: What?

The timing of the fruit's ripening varies

From year to year?

So how do the elephants know when it'll be at its ripest?

Daly: What am I missing here?

What is the elephant in the room?

Schmitt: It's quite obvious that you see first

About an elephant is their trunk,

And their trunk is incredibly interesting

Because you can actually think of it more like a multi-tool.

Narrator: Most importantly, they use it to smell.

And according to science, they are rather good at it.

In 2014, it was discovered that african elephants

Have the best sense of smell of any mammal.

And when we think about other mammals

That have a good sense of smell, let's take dogs, for example.

They have 800 genes responsible for smell.

The african elephant?

2,000 genes.

So we can see that, even from a genetic point of view,

Smell is really important.

In fact, it's thought that this super sense of smell

That elephants have can help them avoid danger.

Narrator: A recent study shows just how incredible

An elephant's ability to smell really is.

For 27 years,

The african country of angola was gripped by bitter civil war.

After that civil war,

The countryside was littered with land mines.

It affected not only humans, but also wildlife.

So it turns out that these elephants were able

To detect the smell of the tnt inside the land miles.

They associated that smell with danger,

And so they avoided those land mines.

Narrator: If elephants can quite literally sniff out danger,

Can they also be using their extraordinary noses

To work out when the mangos inside the lodge are ripe?

Narrator: In zambia, a herd of elephants visits the same hotel each year

To feast on the garden's mangos.

It appears they rely on memory to get there,

But this doesn't explain how they time their arrival

To coincide with the ripening of the fruit.

Scientists have discovered that elephants

Have the strongest sense of smell of any mammal.

Could this be how they know the mangos are ready to eat,

Even if they are many miles away?

Aryee: One of the other important resources

That elephants need is water,

And they can use their sense of smell to find it.

Daly: While water is odorless,

Elephants can smell signs of water.

So chemicals, bacteria, algae, and plant matter,

And as elephants have such good noses,

They can actually smell this water even if it's out of sight.

Schmitt: Elephants are able to do this

Because they're able to maneuver their trunk into the wind

And smell things that are quite a distance away.

Narrator: This is the missing piece of the puzzle.

When the elephants pick up the scent wafting in the wind,

They know the mangos are ready to eat,

Even if they are miles from the hotel at the time.

Aryee: These elephants are using their incredible sense of smell

And coupling that with their incredible memory

To know exactly where to go and when those mangos are ripe.

Narrator: These sweet-toothed elephants have lucked out

With a mammoth memory and superior snout,

Meaning it's mfuwe mangos without a doubt.

This is proof that these animals can really multi-tusk.

♪

Narrator: August 13, 1988.

The north atlantic ocean off the coast of portland, maine.

A group of tourists on a sightseeing trip

Are treated to a spectacular sight.

-Look, look. -Here comes another one.

Whoo!

It's a very interesting video where you see a burst of bubbles

In a ring formation come up to a surface.

Narrator: The circle looks almost solid

Until it reaches the surface

And then disappears.

A few seconds later, another one pops up.

A strange circle of bubbles

Suddenly appear on the surface of the ocean.

What's causing that?

Narrator: Is there a clue to be found beneath the ocean floor?

So when we see this giant bubble coming to the surface,

One possibility is that this is just a giant gas bubble

Maybe coming from some deep-sea vent.

Narrator: Hydrothermal vents occur

In volcanically active regions.

They spew out a constant stream of superheated water and gas.

But gas bubbles are typically spherical.

Could it really be possible for a subsea volcano vent

To belch out giant, circular bubble rings?

Narrator: Off the coast of portland, maine,

Tourists observe a mysterious bubble ring

Rising to the surface of the ocean.

Is it possible that the cause is underwater volcanic activity?

Well, some volcanos aboveground do produce smoke rings,

And the same forces that hold together a smoke ring

In the atmosphere apply underwater.

Schreiber: Could it have been a giant gas bubble

That erupted from the bottom of the ocean,

Rising to the surface?

Most likely not because

You wouldn't get a perfect circle if that happened.

Narrator: So, what is producing these circular bubbles?

Well, soon after the second ring surfaces,

There's another surprise.

So now there's a whale in this story.

Hands up if you saw that one coming.

But what is it doing?

Another possibility is that the ring of bubbles

That we're seeing, it's actually a bubble net

Or a bubble curtain that's being made by certain whale species.

Narrator: Humpback whales use bubble curtains

To hunt their prey.

The complex choreography begins with whales

Diving below a school of fish.

They then swim in a spiral towards the surface

While expelling air from their blow holes.

The fish are too scared to go through the bubbles,

So they stay in, and the whales will get narrower and narrower

Until they're all corralled,

And they can actually go and eat those fish.

Narrator: Were these tourists lucky enough to see firsthand

One of nature's great hunting techniques?

It could be that, sure, but we don't think so

Because they're just -- it's too perfect a circle.

Nosal: If this were bubble nets,

We would probably expect multiple bubbles

To be coming up at different times

Because there's more than one whale participating in this.

Narrator: If the whale in this footage isn't hunting,

What other reason might it have for blowing a giant bubble?

We know that dolphins will also blow these sort of bubble rings,

But on a much smaller scale just for fun.

We know that belugas will do it, as well.

And so what we're seeing with these larger whales

Is basically a larger ring bubble.

They're probably doing the same thing

That these dolphins and belugas are doing.

It's learnt somehow to make this perfect shape

Like blowing smoke rings

That rises to the surface of the ocean.

It's clearly playing around and having fun.

Narrator: Like dolphins, whales are well-known

For their love of social behavior.

And it appears this one has also mastered

The science of fluid mechanics.

These divers show exactly how it works.

Water pressure increase with depth, so as the bubble rises,

The pressure decreases, allowing it to expand.

The deeper the whale goes to blow,

The bigger the bubble can grow.

Nosal: And it turns out that the humpback whale

Blowing these bubbles likes showing off.

He blows bubbles right next to the tourist boats

That are coming to see him.

Narrator: The crew of the whale-watching ship

Who captured this footage made 17,000 trips over 16 years.

Good job they hit record this time.

-- Captions by vitac -- www.Vitac.Com

Captions paid for by discovery communications