Episode 2 Nature's Weirdest Events

No matter how well we think we know our planet, it can still surprise,

shock us and maybe sometimes even 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...

Aargh! There's one on me!

SCREAMING

..and to weather phenomena which have even the experts baffled.

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.

INSECTS HISS

Yes, ma'am, they are loud!

Peaking at over 100 decibels,

the noise was as loud as a rock concert -

one that went on non-stop for five weeks.

And the cause of the racket soon became clear -

the town was under siege from a plague of insects.

The locals were under attack from all directions.

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

SCREAMING

Residents like John G Brittle Junior

got out their video cameras to record the invasion.

'But if you think you got bugs...

'let me tell you something. You ain't got bugs like these!

'We got 'em by the million.'

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.

There are people who don't like bugs.

I quite like bugs, yeah.

'Oh! Hey! Oh!

'They're attacking me - I'm out.'

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.

SCREAMING

So what had caused this plague of almost biblical proportions?

Dr Gene Kritsky has been studying these astonishing swarms.

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'd get kind of tired of it.

And that's what happens with the predators.

And that allows the second wave as they continue to emerge

to have enough 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.

What's interesting is that it's a prime number -

it can only be divided by itself or one.

Now if predators or parasites 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?

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.'

Oooh, what have we got here?

We have a cicada.

There, you see it? Just fell out in my hand.

That is an eight-year-old cicada nymph.

It's seen 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.

Remember the power-tool users getting mobbed by cicadas?

What on earth could be the attraction?

Aargh! What are they?

SCREAMING

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 out-sing 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.

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 the 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?

# And vacate to Vegas

# 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 had 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.

'Oh, this is going to get hairy.

'Uh-oh. 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. Totally going to get him.'

'Watch, watch, watch.'

'Back off, people, back off.'

CRASH 'Ohhh!'

When their ears go back and their eyes get real big

and their head juts out, then you're in trouble.

And they will charge their reflection in windows.

Wildlife officer Rick Spowart and elk enthusiast Jayne Zmijewski

have been closer than most.

They will charge a tree, they will fight bushes,

they will attack swing sets, volleyball nets,

people dive over fences, they run in the water,

they climb trees. Whatever they have to do to get out of the way.

SHOUTING

Every year, we have bull elk

with Christmas tree lights on their antlers.

So what on earth is going on?

What could be driving these huge beasts to attack?

What have the good people of Estes Park done

to deserve such rough treatment?

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

Well, the Estes Valley has probably some of the best elk habitat

that you can find anywhere,

and within this general area,

we have a population that's several thousand elk

that reside here throughout the entire year.

Out of the population, there is a segment of elk

that tend to stay right in and around town.

Essentially, they've grown up and they were born there,

they've spent their entire lives there.

You know, they've become very habituated and accustomed to people.

And they don't see humans as any sort of real threat.

And with plenty of lush grass in constant supply,

the elk have found their perfect home.

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 co-existence.

There's only a couple of times during the year

where all of a sudden we have conflict.

So what's going on?

What could suddenly snap this harmonious co-existence?

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, a big bronze statue

that's about 12 feet 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.

Believe it or not, we're not exempt from this sort of behaviour

right here in the UK. Take a look at this poor chap,

given a right run for his money in a London park.

Dear, oh, dear.

You really wouldn't want to get hit by one of these things.

They are tremendously powerful animals.

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 would you like to spend Christmas

# On Christmas Island? #

But there's one Christmas when you really wouldn't have wanted

to be on Christmas Island.

The beach turned red,

as billions of tiny crabs marched out of the sea

on an unstoppable journey inland.

The entire village was engulfed.

And the onslaught lasted for three solid weeks.

So why had so many miniature crabs swamped the village?

Well, exactly two months earlier,

the islanders had had to deal with this...

..an invasion of much bigger crabs heading in the opposite direction,

towards the beach.

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.

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 20 centimetres across

and they can travel 300 metres an hour.

They may travel anywhere between nine to 15 kilometres,

which is an enormous distance for a crab.

It's the equivalent of marathon distance.

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

so within 28 days, they must complete everything.

And it's a race against time. Out here in the open,

the crabs' biggest threat to survival is drying out.

If the rains stop during the migration,

the migration fails, and tens of thousands of crabs will die.

They will stop to drink on the way any available standing water,

even liquid mud. Should the sun come out,

even though the humidity stays very high,

the crabs will dry out.

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 these 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.

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.

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 that 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.

For our next group of show-stopping events,

we see what happens when we humans

accidentally tip the balance of nature.

From a fearsome army of insect invaders

to an island paradise forced into an eerie silence.

And the escaped pets that have got out of control,

with terrifying consequences.

But we start in Africa, home to red-billed Quelea -

a delicate little bird no bigger than a sparrow.

A member of the artistic weaver bird family,

it showcases its talents

by making an intricate grass nest to attract a mate.

So why would this cute and harmless little bird

be the most feared and hated bird in Africa?

Because it does this.

Huge flocks of Quelea, up to two million strong,

swoop across the landscape turning the sky black.

A flurry of wings all beating furiously.

And when they land to refuel, these ravenous swarms

can get through almost 20 tonnes of grain a day.

Enough to wipe out an entire farm

and cause £40 million worth of damage every year.

They're now one of the world's biggest agricultural pests,

and their devastating appetite has given them

the nickname "feathered locust".

But how do these super-flocks of Quelea get so big?

A single bird may not look like much of a threat,

but it's their habit of hanging around in huge gangs

that's made them a menace.

With a population of 1.5 billion,

Quelea are now the world's most abundant wild bird.

Not bad for a species that's only found in Africa.

Professor Robert Cheke has been trying to find

the key to the Quelea's success.

The breeding process of Quelea birds is really quite spectacular,

and very unusual, because they breed in huge colonies,

and a remarkable thing about the breeding of Quelea birds

is that each female will lay usually about three eggs.

They don't just lay three eggs once a year,

they may lay three eggs three times a year,

or in very rare occasions, up to four or five times a year.

So we're looking at maybe ten chicks per female.

So when you do the maths, that's ten times a million breeding pairs -

potentially ten million new birds a year in just one flock.

But that can't be the whole story,

because not all of these chicks will survive,

and having lots of young is a strategy

that many other species of birds use.

So there must be something else that's helping them thrive.

Experts believe it's connected to their wandering ways.

You see, something that sets them apart from many other birds

is that they are completely nomadic.

So with no permanent home, they simply go where the food is.

Flocks cover tens of thousands of kilometres a year

searching for the best places to feed and nest.

That means following the rain.

The grains that the Quelea feed on

can only germinate after a good soaking,

and rain also helps flush out insects for the chicks to eat.

Downpours in Africa can be few and far between,

but Quelea are experts

at being in the right place at the right time.

Somehow they seem to know where the rain has fallen.

It's possible that they can detect this by physically seeing

clouds and rain formations in the distance,

or they may be able to detect relative humidity

and moisture in the air.

But however they do it,

the Quelea are always in the perfect place to feed.

In the morning, the birds leave en masse,

and it's thought that perhaps some particular birds

which have been very successful the night before in finding food

will act as scouts and guide the other birds,

which are perhaps less efficiently capable of finding their own food,

to particular sites.

It's a winning strategy for the birds -

and the losers are the poor farmers.

Ironically, though, part of the responsibility

for the devastation they cause actually lies with man.

As Africa's farmland continues to expand,

it's providing these birds with fields full of easy pickings,

helping them reach plague-like proportions.

Facing starvation, farmers desperately try to tackle the problem.

But despite killing millions of Quelea every year,

the population bounces right back.

You have to really admire the red-billed Quelea bird,

because it is such a successful and exciting bird.

They seem to have an uncanny ability to survive

whatever humans throw at them.

And despite their destructive side,

there's a real beauty to these birds.

The way those huge flocks of birds so effortlessly change shape

and direction is absolutely spectacular.

And, you know, in the UK, we have a couple of species

that perform similarly aerobatic manoeuvres -

the knot and the starling.

And scientists have always wanted to know

how they coordinate these displays, or murmurations, as we call them.

The latest thinking on how they do it

is that each bird follows its seven closest neighbours,

not just the bird in front.

And it's not so much the distance between them that's important,

but the direction they're travelling in.

By simply sticking to these key rules,

all the birds stay in perfect formation,

whether it's a few thousand knot...

..or a few million Quelea.

The impact that Quelea have on us humans

is partly thanks to the perfect opportunity

that we've given them to expand.

But sometimes we've not just created the right conditions

for an invasion, we've actually helped launch the attack.

We head to the southern United States

on the trail of a tiny but fearless army...

..the South American fire ant.

Voracious predators acting as one coordinated unit,

the ants attack en masse...

..capable of taking down prey many times their own size.

And fuelling their advance,

the queen is able to produce up to 2,000 eggs a day.

Not even floodwater and rivers pose any real barrier.

By interlocking their bodies to create a floating structure,

they can transport the whole colony to safety,

queen, eggs, larvae and all.

Having escaped their native home,

they're now busy setting up colonies all over the globe.

Arriving in the US on a cargo ship in the 1930s

they've become an unstoppable ecological disaster

attacking anything in their path.

Unlike many other ants they have a very special sting.

Using their jaws to grab hold, the other end of their body whips

round to inject their victim with a venom that burns like fire.

But what's worse is the ant now signals to all of the others

that an attack is under way and, within minutes,

a whole army of them show up - all joining in the fight.

It's not long before you're left with a swollen limb

full of angry stings.

Unstoppable, you might think, but as they hit North America's

urban landscape, a very weird Achilles heel was unveiled -

a death wish for electrocution.

You see, they seem to love nothing more

than piling into electrical circuits in their thousands

to meet a crackly fate,

one after the other until the entire electrical system blows.

So why on earth would a seemingly indestructible creature

want to commit mass suicide?

One theory is that the ants are attracted by the magnetic field

surrounding electrical equipment.

Convincing on the face of it, but proof was elusive.

Entomologist and fire ant expert Dr Karen Vail

has been investigating the ants

and their fatal attraction to electricity.

Fire ants spend most time under the ground.

So they don't communicate so much visually as they do through using chemicals.

They use things like alarm pheromones

to indicate a predator's nearby and to recruit other ants

to come and defend the colony.

What this means is that they can respond to danger en masse.

It just takes one ant to signal for the whole colony to respond.

Most people don't realise that they're stepping in a mound

so they have hundreds of ants running up their legs at a time.

That one will bite, release an alarm pheromone,

which causes the others to bite and sting.

It's pretty amazing how organised 240,000 workers can be.

But this doesn't explain why they're drawn

to lethal electrical fields.

We used to think they were attracted to magnetic fields

but what we think happens is that the fire ants are out scouting,

looking for food, and they come across an electrical current

and they get killed.

And they release an alarm pheromone

and when they release the alarm pheromone,

other workers are attracted to that.

They come in, they get killed by the electrical current,

causes more alarm pheromone to be released and before you know it

the majority of the colony has responded to this alarm pheromone

and they end up shorting out equipment.

So rather than electricity drawing the ants in,

it's actually the ants themselves responding to calls of help

from their accidentally frazzled comrades.

We estimate about 6 billion are spent a year

in paying for medical costs, controlling the ants,

replacing livestock, equipment and other damage that they cause.

6 billion! That's an incredible amount of money!

But scientists were at a loss as to how they could tackle the problem.

Electricity was never going to have any meaningful effect

on their numbers.

Then they discovered something a lot more effective

and much, much weirder.

The ants had an even bigger nemesis.

In an incredible twist, the very same pheromone that leads them

to their death in electrical systems also attracts their arch enemy -

a tiny South American fly,

a fraction of the size of the ants themselves.

Found in the ants' original homeland in South America,

the mere presence of this predator can stop them leaving their nests.

The fly is attracted to the alarm pheromones.

It uses them to home in on the ant before injecting

an egg into their body.

The larvae then hatches and grows, feeding on the ant's brain.

When it's ready it secretes an enzyme that dissolves

the connection between the head and body until...

the head pops off.

What a wonderfully weird way to go.

So sci-fi, straight out of the movies.

In the end though, it's the ants' key strength -

their ability to act as one, in synchronicity,

all controlled by their smells -

which is leading to their demise.

You see, the flies are quite literally sniffing them out.

Next, our trail of nature's weirdest invaders takes us

to an island paradise.

The tiny island of Guam in the western Pacific.

Once a haven of island biodiversity,

rich in native species of birds and reptiles.

Its wildlife, having evolved in the absence of wild predators

had little to fear...

..until the 1950s when species started to systematically disappear.

The native bird species simply vanished

and the once raucous rainforest fell completely silent.

What happened next was truly bizarre.

The silent forest filled with spiders,

their webs continually expanding and covering every available space.

So what could have stunned an island paradise into sinister silence?

Well, to answer this we need a quick history lesson about Guam.

Occupying a strategic position in the Western Pacific,

the island was used as a US military base during the Second World War.

But on one of the military machines left behind was a visitor

that would change the face of Guam for ever -

..the Papua New Guinean brown tree snake.

Used to a competitive world, the snake found itself on an island full of food.

And having never seen a predator, the native fauna simply

didn't know how to respond, they were easy pickings.

The snake gradually spread out across the island

decimating native bird populations as it went.

But one species' loss was another species' gain.

With fewer birds to keep them in check

the spider population exploded.

Guam now has 40 times more spiders than its neighbouring islands.

But even the disappearance of their food source

didn't stop their population explosion.

Unusually for snakes the brown tree snake is not a fussy eater.

They're happy to scavenge too.

As a result, their numbers continued to skyrocket.

In the face of the onslaught

the US government tried anything they could think of.

Trapping, searching for the snakes by sight and with dogs.

But confronted with certain defeat they've now decided to deploy

a radical tactic and engage a very covert

and specialist parachute regiment...of mice.

Yes, you heard correctly, lacing dead mice with a chemical

found in paracetamol that is poisonous to the snakes.

They've started to drop them from the air into the forest.

And the snakes' relaxed eating habits means

they readily take the bait.

But as effective as this might be, it's thought it will merely

control the population rather than eradicate the snakes completely.

An accidental invader that's really overstayed its welcome.

But then, not all invasions have been started accidentally.

This brute over here is a cane toad, a resident of South America.

But it was introduced to Australia to try and control the voracious appetite

of the sugar cane beetle.

The trouble is it reneged on its side of the bargain,

and rather than eating the beetles, it's been gobbling up

the native wildlife ever since.

It's a classic case of biological control gone bad.

Other species have been introduced as food,

the most notable example perhaps being this -

the common or garden snail.

This was brought to the UK by the Romans.

We no longer have a taste for these animals

but they do have a taste for things that we grow.

And given that one of them

can produce 480 more in the space of a year they've pretty much

turned themselves into the scourge of the British gardener.

But our next weird invasion is altogether more dangerous

and ruthless than the average snail, and a whole lot more intimidating.

-Got it?

-You all right?

Yeah, go, go, go, go.

In August 2012 the largest Burmese python ever recorded was captured.

Weighing in at just under 75kg, it took four men to wrestle it

out of the bushes.

But this massive Burmese python is nowhere near Burma.

In fact, it's being dragged out of the Florida Everglades.

And worryingly, it's not the only one.

Argh!

Just like on Guam, the local wildlife had never seen

anything like it before, and scientists have recorded

a massive drop of up to 99% in some species of local mammals.

Possums, raccoons and bobcats have all been badly hit.

And even the Everglades' top predator, the alligator,

appears to have met its match.

Clashes between alligators and pythons are common.

So what is a giant snake from South-East Asia doing over 14,000km

away from home?

Well, it's all down to our desire for exotic pets.

Burmese pythons are bought as small and inoffensive snakes

at a manageable length, but this doesn't last long.

The snakes all too often outgrow their owners' ability to keep them.

This gigantic python, with a staggering 76cm girth,

made the headlines when it was removed from one of Florida's

residential neighbourhoods.

Quite capable of eating an adult human,

it was dangerously out of control.

And faced with overgrown snakes many other owners have been known

to set their animals free.

Others simply break out of their cages.

The trouble is that Florida turns out to be their ideal habitat -

plenty of food, water and cover.

And unlike their home in Burma where they're hunted for their skin

and their habitat is in decline, life here is pretty good.

The local wildlife doesn't stand a chance.

Pythons kill their prey by constriction -

squeezing the life out of it and then swallowing it whole.

Like all snakes, they have an incredibly flexible jaw,

enabling them to swallow prey many times the width of their own body.

And as if being able to swallow bigger prey than yourself

wasn't weird enough, after a meal of this size,

the snake won't need to eat again for months.

All together, it's a recipe for success

and the authorities are now removing pythons

from the Everglades in their hundreds.

These weird events show us what happens when man accidentally

interferes with nature's balance and it unleashes its power,

whether it's locust-like plagues of birds,

fearless insects on a global crusade,

or snakes that develop a taste for the local cuisine.

Given the right conditions nature can leave us feeling both powerless

and awestruck.

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.

But 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 light.

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?

It turns out this strange spectacle isn't as rare as you might think.

First spotted way back in 79AD, 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.

LIGHTNING CRACKS

Not easy, when even Graeme Anderson,

one of the top brains at the Met Office,

admits it's a tricky subject.

The way that lightning's generated in normal thunderstorms

isn't completely understood, and lightning from volcanic eruptions

is even rarer and even more treacherous

if you want to get in and try and study it.

Most lightning's 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

and when the charge within the cloud becomes big enough

it can lead to a spark.

That's what you see as lightning, is all of that charge

rushing down a very narrow channel.

It becomes very, very hot,

much hotter than the surface of the sun,

and the air actually explodes which creates the flash of light

and rumble of thunder that you hear.

So it's ice that causes the spark to set off the lightning,

but how on earth can this happen in a volcano where temperatures

can reach over 2,000 degrees Fahrenheit?

Look at the debris there. Large pieces of rock and lightning.

So much kinetic energy being released.

Stay here. Can you stay?

-I can't, the wind is just too strong.

-OK.

Well, one theory that attempts to explain it

is the Dirty Thunderstorm Hypothesis.

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 on 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 really pump a lot of moisture into the upper atmosphere,

several kilometres up 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 in 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.

This is catastrophic. Whoa, 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 sub-flashes?

And they're 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 millionths 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.

It's an amazing cloud formation here in Burketown. Wow!

Row after row of long tube-like clouds

stretching from horizon to horizon.

The phenomenon had 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 then as it passes over, momentarily the sky becomes overcast

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

the sky clears again and with that movement, you get the rushing winds

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

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

So it's quite an experience when one of these Morning Glory clouds passes over you.

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.

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 unexplainable skies.

It's a very strange-looking cloud formation.

But when it comes to causing alarm, few clouds can compete

with the 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 cloud is known as the king of clouds

and this is because of its size

and because of the sort of weather it produces.

And this is the extreme end of our weather.

The energy within one of these enormous cumulonimbus clouds

is equivalent to the energy of ten atom bombs.

Just amazing!

But some clouds don't just look catastrophic

they look almost other-worldly.

These strange saucers have had many people looking

for supernatural explanations.

Lenticularis clouds.

You would most often find them in the vicinity of mountains

because the process that causes them to form is all to do with the wind,

the air having to rise to pass over a mountain.

Downwind from the peak, the air can take on this wavelike path,

rising and dipping.

At the peak of these waves, hovering in the wind,

is one of these lenticular clouds.

They can look remarkably like UFOs.

Like disc-shaped.

Sometimes they're stacked so you have one disc here

and then almost a little gap and then a disc on top,

which looks rather like the sort of pod that the aliens would sit in, I suppose.

But there are still some clouds that even the experts

find hard to explain.

On the 26th of June 2012 the skies over Regina in Canada

brought residents running out into the streets.

We've experienced one of the coolest things

I don't think I'll experience again in my life.

We just experienced a wicked storm and now the sky

has got these little puffy bubble marshmallowy things going on.

And I probably will never experience this again in my life.

The mama cloud, which is also known as mammatus,

when they cover the sky, it looks almost like an Independence Day, sort of doomsday situation.

You think, "My word, what are those about?"

They're so kind of globular and dramatic.

You do often see great examples out in the great plains of the States.

You know, Tornado Alley. All the places where the stormchasers go.

Quite how mama clouds form is not really clear.

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

they're so omnipresent 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 it would weigh a million tonnes.

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

or 6,200 blue whales - all just hovering right 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?

-Whoa!

Subtitles by Red Bee Media Ltd