Episode 2 Nature's Weirdest Events

No matter how well we think we know our planet,

the natural world still has the ability to surprise us,

to shock us and maybe sometimes even to scare us

with its extraordinary events and bizarre behaviour.

And new technology means that nature's weirdest phenomena

are being caught ever more readily on camera.

So we're going to bring you the strangest stories

our world has to offer...

..from an island where the locals are awash with crabs...

to the residents overwhelmed by a deafening plague of insects.

Argh! There's one on me!

SHE SCREAMS

With the help of scientists, experts and eyewitnesses,

we're going to try and unravel exactly what on earth is going on.

Nature often has the power to amaze us,

but sometimes it can feel that it's gone just a bit too far,

stopping us in our tracks with events that are impossible

to ignore, and we start with animal invasions

so shocking that they disrupt, disturb and suspend normal life,

from the elk running riot to the crabs on an unstoppable mission.

But we start in America, in Nashville, Tennessee,

where every spring the air softly buzzes with the sound of insects.

But in 2011, this gentle chorus turned into a deafening roar.

LOUD CHORUS OF CHIRRUPING

-Yes, ma'am, they are loud!

-Oh!

MAN LAUGHS

Peaking at over 100 decibels, the noise was as loud as a rock concert,

one that went on nonstop for five weeks.

But the cause of the racket soon became clear.

CHIRRUPING CONTINUES

The town was under siege from a plague of insects.

The locals were under attack from all directions.

STRIMMER BUZZES

And anyone using a power tool outside was being completely mobbed.

SCREAMING

Residents like John G Brittle Jr

got out their video cameras to record the invasion.

If you think you got bugs...

let me tell you something.

You ain't got bugs like these.

We got 'em...in the millions.

And the insects in question were cicadas, a completely harmless

but very vocal relation to the aphid.

It's noisy. There's just this din all the time, a hum.

'It can be pretty scary.

'They're literally flying around and you're batting them away

'and trying to get them out of your hair.

'I have friends who didn't leave their houses.'

WOMAN SCREAMS

'There are people who don't like bugs.'

I quite like bugs, yeah.

Oh, Amy! Oh, whoa!

INDISTINCT

The invasion started on a warm spring evening in May.

All over town, wave after wave

of cicadas crawled their way

out of the earth.

In just five days, almost ten million of them

had formed a ghostly, red-eyed army.

One by one, they moulted out of their old skins...

..which remained clinging eerily to the trees.

Their arrival brought the town to a standstill.

SHE SCREAMS

So what had caused this plague of almost biblical proportions?

Dr Gene Kritsky has been studying these astonishing swarms.

The first few that come out and transform into the adults

usually get eaten by birds and squirrels and raccoons, but

eventually, the predators become so tired of eating them, they just stop.

To use an illustration, if you were to go outside and see

the place being riddled with hundreds of chocolate candies,

you might eat as many as you can, but eventually you get

kind of tired of it, and that's what happens to the predators.

And that allows the second wave, as they continue to emerge,

to have enough for individuals around to survive to reproduce.

Now, these incredibly high numbers of insects are weird enough,

but there was also something truly extraordinary going on -

these invasions were happening as regular as clockwork every 13 years.

What could possibly cause this bizarre 13-year pattern?

Scientists discovered that these weren't the usual annual cicadas

that Tennessee was used to.

This was a totally different species,

known as a periodical cicada -

an insect that only emerges in plague-like numbers every 13 years.

Clearly, the number 13 must be pretty important,

but what's interesting is that it's a prime number -

it can only be divided by itself or one.

Now, if parasites or predators have a different type of annual

cycle - say two, three or four years -

their peak emergence will never coincide with that of the cicadas.

Therefore, the cicadas will have a greater chance of survival -

proving that for at least some species,

the number 13 is far from unlucky.

But it seems like a weird life, waiting underground

for so many years.

So what exactly is going on down there?

So what I'm looking for right now are cicada nymphs, immatures

that will be inside these little clusters of dirt,

and they're not sleeping -

they're actually digging along a tree root, feeding, growing,

moulting and getting ready for their emergence.

They only emerge from the ground to transform into the adult

and sing, mate, lay eggs and die.

Oh! What have we got here?

We have a cicada.

There you see it.

It just fell onto my hand.

That is an eight-year-old

cicada nymph, seeing light that it wasn't expecting to see,

and it digs with rather enlarged forelegs,

which it's using right now to crawl on my hand.

Finally, in their 13th year, they're ready to crawl up into the light.

The mission - to find a mate and breed.

But to do this, it needs an important last-minute addition -

wings.

But cicadas of the opposite sex weren't the only things

getting their attention.

BUZZING

Remember the power-tool users getting mobbed by cicadas?

What on earth could be the attraction?

Argh! There's one on me!

STRIMMER BUZZES

SCREAM

Well, the answer might be linked to the reason that they

invaded the town in the first place.

Males gather in trees in large numbers -

we call them chorusing centres.

It's almost like a periodical cicada singles bar.

Here, they all try to outsing each other, hoping to win

the affections of the females.

But vibrating at much the same frequency as the males'

deafening love song is your average power tool.

BUZZING

It's this that explains the fatal attraction.

And for a lot of people, the affection goes both ways.

The residents of Tennessee and all the other places

they visit have taken cicadas to their hearts,

celebrating their arrival.

# The cicadas are invading our state of Tennessee

# Hatching and chillaxing on everything they see... #

# The cicadas

# Why can't you leave us all alone?

# Vacate to Vegas

# And say farewell to this time zone

# Oh, cicadas

# Why don't you... #

But, actually, it's not just 13-year cicadas.

In other parts of the United States, there's another type of cicada

which emerges every 17 years.

And, amazingly, that's another prime number, so clearly the same

survival strategy is working for this species, too.

Now, whether you love or loathe cicadas, there is

one date you should put in your diary.

Every 221 years,

all of these animals will emerge at the same time.

There's just one bit of bad news -

I'm afraid that the next time this is going to happen in Tennessee...

is in 2076.

But if Tennessee thought it was struggling to cope

with its plagues of insects, that's nothing compared to what

one town have to put up with from another amorous animal.

Estes Park, Colorado.

Nestled amongst the stunning scenery of the Rocky Mountain National Park.

A peaceful wilderness retreat where nature lovers and wildlife

can live cheek by jowl in harmonious equilibrium.

Until the autumn, when everything changes.

ELK BELLOWS

This is going to get hairy.

Uh-oh! No! No!

The elk in Estes Park suddenly flip, attacking anything that moves

and becoming a danger to the local residents and themselves.

It's going to get him.

Watch, watch, watch.

CRASH!

Back up, people! Back up!

Wildlife consultant Chris Rowe knows all about the elk of Estes.

In the town you essentially have two different populations living there.

You've got the population of people and you've got the population of elk.

For most of the year it's a pretty peaceful coexistence.

There's only a couple of times during the year where all of a sudden

we have conflict.

PEOPLE SHOUT OUT

So what's going on?

What could suddenly snap this harmonious coexistence?

ELK BELLOW

Well, just like the red deer we have in Britain, elk have one

period each year when they get a bit more hot and bothered.

During the rut the males' bodies are pumped with testosterone.

They spend every bit of energy defending their patch

and attracting a harem of females.

As we move into the month of September,

that testosterone level ramps up and their aggression and their

intensity level on protecting those cows REALLY, really ramps up.

They're not afraid to lock antlers and get physical.

Given the fact that Estes sits right smack-dab in the middle

of some of the most perfect habitat, all this occurs right in town,

in and around the houses, in and around the vehicles,

right in the middle of the street a lot of times.

The males are so driven that they don't

differentiate between threats to their dominance.

Anything that moves is fair game.

One of the newspaper people got a picture of a young bull attacking

Samson, who's a big bronze statue that's about 12ft tall at least.

With these human-habituated elk right around people in Estes,

there's not really a usual day.

I've taken a bicycle off of an elk, garbage-can lids,

every kind of fencing material you can imagine.

So, for two months a year,

the normally peaceful Estes Park fills with chaos and disruption.

Enormous, rampant deer are one thing,

but when it comes to disruption,

sometimes it's the little things that pack the biggest punch.

Our trail of extraordinary animal invasions now leads us

to Christmas Island in the Indian Ocean.

The island looks like the perfect tropical paradise.

# How'd you like to spend Christmas

# On Christmas Island? #

These are a species called Christmas Island crabs

and they're on their annual migration from their home

in the rainforest to the water's edge.

They may be land crabs but they still need to lay

their eggs in the sea.

THUNDER CLAPS

So, once a year, the arrival of the monsoon rains prompts

the crabs to emerge from all over the forest.

Crab expert Dr Simon Webster has been

studying this perfectly timed march to the water's edge.

They're tiny animals, they're only 20cm across,

and they can travel 300 metres an hour.

They may travel anywhere between 9-15km, which is

an enormous distance for a crab.

It's the equivalent of a marathon distance.

They must migrate, mate and spawn within one lunar cycle,

so within 28 days they must complete everything.

As they emerge from the rainforest,

they instinctively know which way to go.

It's the same well-trodden path that their ancestors have been

taking for generations.

It's a route that takes them straight through the village

and headlong into the everyday lives of the long-suffering humans here.

Roads are carpeted with red

and millions upon millions of those crabs come out of the rainforest,

cross roads, go through people's houses down to the sea.

It is one of the most spectacular animal migrations on Earth.

These crabs are very good climbers.

They can climb up the corner of a room quite easily.

CAT MEOWS

And they will go through any doorway, wardrobe...

They will end up in drawers, in sinks, anywhere.

Having run the gauntlet of all of the human obstacles,

the crabs arrive at the beach exhausted.

But there's only time for a quick dip to refresh their parched

bodies before their thoughts turn quickly to mating

and releasing their eggs into the sea.

Once they're set adrift, the crab larvae are completely

dependent on the movements of the tides.

And it's a dangerous world.

Most years these babies end up as fish food, or get swept out and

lost in deep waters, but once, maybe twice a decade, they get lucky.

The tide brings vast numbers of tiny crabs back to the shore to

begin their march into the forest.

The small crabs, when they emerge, are just a few

millimetres across - about half the size of a small fingernail.

They're almost transparent, you can see the organs within them.

They are very, very delicate.

They have the same sort of texture as a pea.

So they're very easy to crush.

The islanders take great care not to kill any animals at all.

There are underpasses on the roads, roads are closed,

there are even a couple of crab bridges.

It's hard to imagine how something so fragile can survive the journey.

But having completed this amazing migration back to the shore,

what happens next is actually a bit of a mystery.

You see, just a month after the first tiny crabs

arrive on the beach, they all disappear back into the rainforest,

millions and millions of them, all going their own separate ways

and living in isolation.

What all these stories show is when it comes to reproduction,

nature is a force to be reckoned with.

Whether it's a plague of insects looking for love...

..a mob of amorous elk scaring off the competition,

or an invasion of crabs caused by a race to mate,

when we get in the way of nature's need to breed,

it can stop us in our tracks.

In this next section,

we move from the devastating power of biological onslaughts...

..to atmospheric ones

and to some of the most spectacular meteorological

mysteries on the planet...

..from apocalyptic clouds...

That is one crazy-looking storm.

..to lightning that's lost its storm.

And for that story, we travel to Iceland.

April, 2010.

A volcano that had been dormant for 200 years suddenly erupted.

This is as close as we dare go to this huge plume.

As lava oozed out down the mountain, huge plumes of ash

were sent skywards, reaching heights of over 10km.

It's when the ash cloud started spreading out across

Northern Europe that the eruption really hit the headlines.

UK airspace is closed for the first time.

All flights have been grounded amid safety fears

as a cloud of volcanic ash drifts over Britain.

And with all eyes trained on that ash cloud,

people started to notice freak flashes of light.

Whoa! Look at the lightning up there.

There wasn't a storm cloud in sight,

but bolts of lightning were coming in thick and fast.

So why on earth was a volcano alive with lightning?

Well, it turns out this strange spectacle isn't as rare as

you might think.

First spotted way back in 79 AD,

it's been making the occasional mystifying appearance ever since.

But to work out what causes it,

first we need to get to grips with lightning.

Oh!

THUNDER CRASHES AND LIGHTNING STRIKES

Not easy when even Graham Anderson,

one of the top brains at the Met Office,

admits it's a tricky subject.

The way that lightning is generated in normal thunderstorms

isn't completely understood.

Lightning from volcanic eruptions is even rarer

and even more treacherous if you want to get in and try and study it.

Most lightning is created within shower clouds and thunderclouds.

Those clouds are created when the atmosphere is said to be unstable.

In these stormy conditions,

moisture in the air is drawn high up into the clouds.

Here, it reaches temperatures so cold

that the water droplets turn to ice.

And this is where the physics gets weird,

because to get lightning, which has a temperature hotter than

the surface of the sun, you actually need large quantities of ice.

Those ice particles are bouncing and rubbing off each other and over time,

this leads to a generation of charge that spreads out within the cloud.

When the charge within the cloud becomes big enough,

it can lead to a spark.

And that is what you see as lightning,

all of that charge rushing in a very narrow channel.

The lightning within a volcanic ash plume

is generated from moisture emitted from the volcano,

rising up into the atmosphere and condensing into water droplets,

that then carry upwards and freeze, creating ice particles.

You need a particularly vigorous eruption,

one that's going to really throw out a lot of heat and energy

and pump a lot of moisture into the upper atmosphere,

several kilometres into the air,

so that it reaches those cold levels of the atmosphere.

Not all volcanoes will have enough energy

to get water high enough to freeze,

which is what makes volcanic lightning so unpredictable.

But as long as you have ice,

you have your key ingredient for lightning.

That process of generating lightning in the same way

as a thunderstorm can work within a volcanic ash plume.

Regions of charge within the plume will lead to a spark,

which is the lightning strike that you can see.

-RADIO:

-This is catastrophic.

Wow, look at the lightning there.

Dramatic stuff!

But did you know that what we perceive as a single

bolt of lightning could be composed of up to as many as 25

super-fast subflashes

and they are so quick that we still see it as a single bolt?

And they're not only fast, they're also incredibly intense

and incredibly bright,

so much so that they burn an image onto our eyes

that lasts for several seconds

despite the fact that their combined duration is less

than 100 millionth of one second.

But weather doesn't always have to be loud and flashy

to have us stopped in our tracks.

Sometimes, even a silky sky presents us

with events that are more science fiction than science.

In Burketown, Australia, every September,

residents wake up to these incredible scenes.

This amazing cloud formation here in Burketown. Wow!

Row after row of long, tube-like clouds,

stretching from horizon to horizon.

The phenomenon has been dubbed the Morning Glory by the locals.

And every year, as spring arrives, so do the clouds.

So, what brings this Morning Glory to Burketown?

Gavin Pretor-Pinney, a dedicated cloud spotter,

has spent a lifetime staring skywards.

The clouds are expressions on the face of the atmosphere

and they can be read, like the expressions on the face of a person.

They reveal the moods of the atmosphere.

When it arrives, this cloud looks very dramatic.

The sky is clear, you see this tube rolling along towards you

and as it passes over, momentarily, the sky becomes overcast,

and then as it moves on, and you're left in its wake,

the sky clears again.

With that movement, you get the rushing winds as it approaches,

and then once it's over you, the wind momentarily drops

and as it passes on, the wind picks up again.

So it's quite an experience

when one of these Morning Glory clouds passes over.

OK, if clouds are trying to tell us something about our skies,

what on earth is the Morning Glory saying to us?

The peninsula gets heated up by the sun during the day.

The sea breezes come in, both sides collide

and set off this wave which travels through the night,

arriving at Burketown, and within that wave of air,

a roll of cloud can form.

And although no other place puts on quite such a spectacular

show as Burketown, you do get the odd show-stopping single roll cloud

appearing in other coastal areas.

Oh, what is it? It's right above us.

And these aren't the only clouds that have had us mystified.

Across the globe, people have rushed out

to record their own seemingly inexplicable skies.

It's a very strange-looking cloud formation.

But when it comes to causing alarm,

few clouds can compete with a cumulonimbus.

It looks a little bit like an atom bomb cloud, a mushroom cloud.

It can reach 10, 12 miles up into the sky.

The cumulonimbus is known as the King of Clouds.

This is because of its size

and because of the sort of weather it produces.

This is the extreme end of our weather.

We're so used to seeing these different cloud forms above us,

they're so omnipresent, clouds, and yet the chaotic movements

of the atmosphere make them really rather difficult

sometimes to understand.

As a child, I also had a fascination with those atomic bomb clouds,

although, I must confess, in a rather dark way.

But what I didn't realise then was that clouds are heavy, very heavy.

Take the average fluffy cumulus cloud.

Using the volume, the air density and the concentration of water

droplets, we can calculate that it might weigh a million tonnes.

That's the same as 200,000 African bull elephants,

or 6,200 blue whales, all just hovering above our heads.

These weird weather events show us that the natural world

still has the power to surprise and keep us guessing,

whether it's bolts of light fired down through the ash,

or clouds that make us feel like we're under siege from above.

What all of these stories seem to tell us

is that sometimes you can't ignore the full force of nature.

So, whether we're just trying to live alongside it,

or perhaps even contain it, it's bound to throw up a few surprises,

curious and baffling events that are sure to have us sitting up

and staring in amazement.

Next time on Nature's Weirdest Events:

There's the mystery of oozing sea slime.

Bizarre body snatchers.

And a butterfly blizzard.

Do you see that, guys? Wow!