0:00:02 > 0:00:06No matter how well we think we know our planet, it can still surprise,
0:00:06 > 0:00:10shock us and maybe sometimes even scare us
0:00:10 > 0:00:14with its extraordinary events and bizarre behaviour.
0:00:14 > 0:00:18And new technology means that nature's weirdest phenomena
0:00:18 > 0:00:21are being caught ever more readily on camera.
0:00:21 > 0:00:23So we're going to bring you
0:00:23 > 0:00:26the strangest stories our world has to offer.
0:00:27 > 0:00:32From an island where the locals are awash with crabs
0:00:32 > 0:00:36to the residents overwhelmed by a deafening plague of insects...
0:00:36 > 0:00:38Aargh! There's one on me!
0:00:38 > 0:00:40SCREAMING
0:00:44 > 0:00:48..and to weather phenomena which have even the experts baffled.
0:00:49 > 0:00:53With the help of scientists, experts and eyewitnesses,
0:00:53 > 0:00:57we're going to try and unravel exactly what on earth is going on.
0:01:19 > 0:01:23Nature often has the power to amaze us,
0:01:23 > 0:01:26but sometimes it can feel that it's gone just a bit too far,
0:01:26 > 0:01:29stopping us in our tracks with events
0:01:29 > 0:01:31that are impossible to ignore.
0:01:31 > 0:01:35And we start with animal invasions so shocking that they disrupt,
0:01:35 > 0:01:39disturb and suspend normal life,
0:01:39 > 0:01:41from the elk running riot
0:01:41 > 0:01:44to the crabs on an unstoppable mission.
0:01:46 > 0:01:49But we start in America, in Nashville, Tennessee,
0:01:49 > 0:01:54where every spring, the air softly buzzes with the sound of insects.
0:01:54 > 0:02:00But in 2011, this gentle chorus turned into a deafening roar.
0:02:00 > 0:02:03INSECTS HISS
0:02:03 > 0:02:04Yes, ma'am, they are loud!
0:02:07 > 0:02:10Peaking at over 100 decibels,
0:02:10 > 0:02:13the noise was as loud as a rock concert -
0:02:13 > 0:02:16one that went on non-stop for five weeks.
0:02:17 > 0:02:19And the cause of the racket soon became clear -
0:02:23 > 0:02:26the town was under siege from a plague of insects.
0:02:28 > 0:02:31The locals were under attack from all directions.
0:02:32 > 0:02:38And anyone using a power tool outside was being completely mobbed.
0:02:39 > 0:02:41SCREAMING
0:02:42 > 0:02:45Residents like John G Brittle Junior
0:02:45 > 0:02:48got out their video cameras to record the invasion.
0:02:48 > 0:02:50'But if you think you got bugs...
0:02:50 > 0:02:54'let me tell you something. You ain't got bugs like these!
0:02:54 > 0:02:57'We got 'em by the million.'
0:02:58 > 0:03:01And the insects in question were cicadas,
0:03:01 > 0:03:05a completely harmless but very vocal relation to the aphid.
0:03:05 > 0:03:10It's noisy. There's just this din all the time, a hum.
0:03:12 > 0:03:14It can be pretty scary.
0:03:14 > 0:03:17They're literally flying around and you're batting them away
0:03:17 > 0:03:18and trying to get them out of your hair.
0:03:18 > 0:03:21I have friends who didn't leave their houses.
0:03:21 > 0:03:23There are people who don't like bugs.
0:03:23 > 0:03:25I quite like bugs, yeah.
0:03:25 > 0:03:29'Oh! Hey! Oh!
0:03:29 > 0:03:31'They're attacking me - I'm out.'
0:03:31 > 0:03:35The invasion started on a warm spring evening in May.
0:03:37 > 0:03:40All over town, wave after wave of cicadas
0:03:40 > 0:03:43crawled their way out of the earth.
0:03:43 > 0:03:46In just five days, almost ten million of them
0:03:46 > 0:03:49had formed a ghostly red-eyed army.
0:03:56 > 0:04:00One by one they moulted out of their old skins...
0:04:02 > 0:04:04..which remained clinging eerily to the trees.
0:04:09 > 0:04:13Their arrival brought the town to a standstill.
0:04:18 > 0:04:19SCREAMING
0:04:21 > 0:04:25So what had caused this plague of almost biblical proportions?
0:04:28 > 0:04:32Dr Gene Kritsky has been studying these astonishing swarms.
0:04:34 > 0:04:36First few that come out and transform into the adults
0:04:36 > 0:04:39usually get eaten by birds and squirrels and raccoons.
0:04:39 > 0:04:43But eventually, the predators become so tired of eating them,
0:04:43 > 0:04:45they just stop.
0:04:46 > 0:04:49To use an illustration, if you were to go outside
0:04:49 > 0:04:53and see the place being riddled with hundreds of chocolate candies,
0:04:53 > 0:04:54you might eat as many as you can,
0:04:54 > 0:04:56but eventually you'd get kind of tired of it.
0:04:56 > 0:04:59And that's what happens with the predators.
0:04:59 > 0:05:01And that allows the second wave as they continue to emerge
0:05:01 > 0:05:05to have enough individuals around to survive to reproduce.
0:05:06 > 0:05:10Now these incredibly high numbers of insects are weird enough,
0:05:10 > 0:05:14but there was also something truly extraordinary going on.
0:05:14 > 0:05:17These invasions were happening as regular as clockwork,
0:05:17 > 0:05:20every 13 years.
0:05:24 > 0:05:28What could possibly cause this bizarre 13-year pattern?
0:05:28 > 0:05:32Scientists discovered that these weren't the usual annual cicadas
0:05:32 > 0:05:35that Tennessee was used to.
0:05:35 > 0:05:40This was a totally different species known as a periodical cicada...
0:05:41 > 0:05:46..an insect that only emerges in plague-like numbers every 13 years.
0:05:48 > 0:05:51Clearly, the number 13 must be pretty important.
0:05:51 > 0:05:54What's interesting is that it's a prime number -
0:05:54 > 0:05:57it can only be divided by itself or one.
0:05:57 > 0:06:01Now if predators or parasites have a different type of annual cycle,
0:06:01 > 0:06:04say, two, three or four years,
0:06:04 > 0:06:08their peak emergence will never coincide with that of the cicadas.
0:06:08 > 0:06:12Therefore, the cicadas will have a greater chance of survival,
0:06:12 > 0:06:15proving that for at least some species,
0:06:15 > 0:06:17the number 13 is far from unlucky.
0:06:19 > 0:06:21But it seems like a weird life,
0:06:21 > 0:06:23waiting underground for so many years.
0:06:25 > 0:06:27So what exactly is going on down there?
0:06:29 > 0:06:32What I'm looking for right now are cicada nymphs,
0:06:32 > 0:06:35immatures, that will be inside these little clusters of dirt.
0:06:35 > 0:06:39And they're not sleeping. They're actually digging along a tree root,
0:06:39 > 0:06:42feeding, growing, moulting, and getting ready for their emergence.
0:06:42 > 0:06:47'They only emerge from the ground to transform into the adult,
0:06:47 > 0:06:51'and sing, mate, lay eggs, and die.'
0:06:51 > 0:06:53Oooh, what have we got here?
0:06:54 > 0:06:57We have a cicada.
0:06:57 > 0:07:00There, you see it? Just fell out in my hand.
0:07:00 > 0:07:02That is an eight-year-old cicada nymph.
0:07:02 > 0:07:05It's seen light that it wasn't expecting to see.
0:07:05 > 0:07:08And it digs with rather enlarged forelegs,
0:07:08 > 0:07:12which it's using right now to crawl on my hand.
0:07:14 > 0:07:16Finally, in their 13th year,
0:07:16 > 0:07:18they're ready to crawl up into the light.
0:07:19 > 0:07:23The mission - to find a mate and breed.
0:07:24 > 0:07:28But to do this it needs an important last-minute addition -
0:07:28 > 0:07:29wings.
0:07:33 > 0:07:35But cicadas of the opposite sex
0:07:35 > 0:07:37weren't the only things getting their attention.
0:07:38 > 0:07:42Remember the power-tool users getting mobbed by cicadas?
0:07:42 > 0:07:44What on earth could be the attraction?
0:07:46 > 0:07:48Aargh! What are they?
0:07:51 > 0:07:53SCREAMING
0:07:53 > 0:07:56Well, the answer might be linked to the reason
0:07:56 > 0:07:59that they invaded the town in the first place.
0:07:59 > 0:08:01Males gather in trees in large numbers.
0:08:01 > 0:08:03We call them chorusing centres.
0:08:03 > 0:08:06It's almost like a periodical cicada singles bar.
0:08:06 > 0:08:09Here they all try to out-sing each other,
0:08:09 > 0:08:12hoping to win the affections of the females.
0:08:13 > 0:08:16But vibrating at much the same frequency
0:08:16 > 0:08:20as the males' deafening love song is your average power tool.
0:08:20 > 0:08:23It's this that explains the fatal attraction.
0:08:44 > 0:08:48And for a lot of people, the affection goes both ways.
0:08:48 > 0:08:51The residents of Tennessee and all the other places they visit
0:08:51 > 0:08:54have taken the cicadas to their hearts,
0:08:54 > 0:08:56celebrating their arrival.
0:08:57 > 0:09:00# The cicadas are invading our state of Tennessee
0:09:00 > 0:09:03# Hatching and chillaxing on everything they see... #
0:09:03 > 0:09:05# The cicadas
0:09:05 > 0:09:08# Why can't you leave us all alone?
0:09:08 > 0:09:11# And vacate to Vegas
0:09:11 > 0:09:14# Say farewell to this time zone
0:09:14 > 0:09:16# Oh, cicadas
0:09:16 > 0:09:17# Why don't you...? #
0:09:17 > 0:09:21But actually, it's not just 13-year cicadas.
0:09:21 > 0:09:23In other parts of the United States,
0:09:23 > 0:09:27there's another type of cicada which emerges every 17 years -
0:09:27 > 0:09:30and amazingly, that's another prime number.
0:09:30 > 0:09:35So clearly the same survival strategy is working for this species too.
0:09:35 > 0:09:38Now, whether you love or loathe cicadas,
0:09:38 > 0:09:40there is one date you should put in your diary.
0:09:40 > 0:09:43Every 221 years,
0:09:43 > 0:09:46all of these animals will emerge at the same time.
0:09:46 > 0:09:49There's just one bit of bad news.
0:09:49 > 0:09:52I'm afraid that the next time this is going to happen in Tennessee
0:09:52 > 0:09:54is in 2076.
0:09:57 > 0:10:00But if Tennessee thought it was struggling to cope
0:10:00 > 0:10:02with its plagues of insects,
0:10:02 > 0:10:05that's nothing compared to what one town had to put up with
0:10:05 > 0:10:07from another amorous animal.
0:10:08 > 0:10:10Estes Park, Colorado.
0:10:13 > 0:10:18Nestled amongst the stunning scenery of the Rocky Mountain National Park.
0:10:21 > 0:10:25A peaceful wilderness retreat where nature lovers and wildlife
0:10:25 > 0:10:29can live cheek by jowl in harmonious equilibrium.
0:10:31 > 0:10:35Until the autumn, when EVERYTHING changes.
0:10:37 > 0:10:39'Oh, this is going to get hairy.
0:10:39 > 0:10:41'Uh-oh. Uh-oh! No! No!'
0:10:44 > 0:10:49The elk in Estes Park suddenly flip, attacking anything that moves,
0:10:49 > 0:10:54and becoming a danger to the local residents and themselves.
0:10:54 > 0:10:56'It's going to get him. Totally going to get him.'
0:10:58 > 0:10:59'Watch, watch, watch.'
0:11:05 > 0:11:07'Back off, people, back off.'
0:11:09 > 0:11:11CRASH 'Ohhh!'
0:11:13 > 0:11:16When their ears go back and their eyes get real big
0:11:16 > 0:11:19and their head juts out, then you're in trouble.
0:11:20 > 0:11:24And they will charge their reflection in windows.
0:11:26 > 0:11:31Wildlife officer Rick Spowart and elk enthusiast Jayne Zmijewski
0:11:31 > 0:11:32have been closer than most.
0:11:32 > 0:11:35They will charge a tree, they will fight bushes,
0:11:35 > 0:11:39they will attack swing sets, volleyball nets,
0:11:39 > 0:11:42people dive over fences, they run in the water,
0:11:42 > 0:11:46they climb trees. Whatever they have to do to get out of the way.
0:11:46 > 0:11:48SHOUTING
0:11:50 > 0:11:52Every year, we have bull elk
0:11:52 > 0:11:54with Christmas tree lights on their antlers.
0:11:56 > 0:11:59So what on earth is going on?
0:11:59 > 0:12:03What could be driving these huge beasts to attack?
0:12:03 > 0:12:06What have the good people of Estes Park done
0:12:06 > 0:12:07to deserve such rough treatment?
0:12:11 > 0:12:16Wildlife consultant Chris Roe knows all about the elk of Estes.
0:12:16 > 0:12:20Well, the Estes Valley has probably some of the best elk habitat
0:12:20 > 0:12:21that you can find anywhere,
0:12:21 > 0:12:23and within this general area,
0:12:23 > 0:12:26we have a population that's several thousand elk
0:12:26 > 0:12:30that reside here throughout the entire year.
0:12:31 > 0:12:35Out of the population, there is a segment of elk
0:12:35 > 0:12:39that tend to stay right in and around town.
0:12:39 > 0:12:42Essentially, they've grown up and they were born there,
0:12:42 > 0:12:43they've spent their entire lives there.
0:12:43 > 0:12:47You know, they've become very habituated and accustomed to people.
0:12:47 > 0:12:50And they don't see humans as any sort of real threat.
0:12:52 > 0:12:56And with plenty of lush grass in constant supply,
0:12:56 > 0:12:58the elk have found their perfect home.
0:12:58 > 0:13:02In the town, you essentially have two different populations living there.
0:13:02 > 0:13:05You've got the population of people, and you've got the population of elk.
0:13:07 > 0:13:10For most of the year, it's a pretty peaceful co-existence.
0:13:10 > 0:13:12There's only a couple of times during the year
0:13:12 > 0:13:14where all of a sudden we have conflict.
0:13:17 > 0:13:19So what's going on?
0:13:19 > 0:13:22What could suddenly snap this harmonious co-existence?
0:13:26 > 0:13:29Well, just like the red deer we have in Britain,
0:13:29 > 0:13:31elk have one period each year
0:13:31 > 0:13:33when they get a bit more hot and bothered.
0:13:35 > 0:13:40During the rut, the males' bodies are pumped with testosterone.
0:13:40 > 0:13:43They spend every bit of energy defending their patch
0:13:43 > 0:13:45and attracting a harem of females.
0:13:47 > 0:13:49As we move into the month of September,
0:13:49 > 0:13:52that testosterone level ramps up,
0:13:52 > 0:13:54and their aggression and their intensity level
0:13:54 > 0:13:59on protecting those cows really, really ramps up.
0:13:59 > 0:14:02They're not afraid to lock antlers and get physical.
0:14:04 > 0:14:07Given the fact that Estes sits right smack dab in the middle of
0:14:07 > 0:14:10some of the most perfect habitat,
0:14:10 > 0:14:13all this occurs right in town, in and around the houses,
0:14:13 > 0:14:14in and around the vehicles,
0:14:14 > 0:14:17right in the middle of the street a lot of times.
0:14:19 > 0:14:23The males are so driven that they don't differentiate
0:14:23 > 0:14:26between threats to their dominance.
0:14:26 > 0:14:28Anything that moves is fair game.
0:14:31 > 0:14:35One of the newspaper people got a picture of a young bull
0:14:35 > 0:14:39attacking Samson, a big bronze statue
0:14:39 > 0:14:42that's about 12 feet tall at least.
0:14:42 > 0:14:46With these human-habituated elk right around people in Estes,
0:14:46 > 0:14:48there's not really a usual day.
0:14:48 > 0:14:51I've taken a bicycle off of an elk,
0:14:51 > 0:14:55garbage-can lids, every kind of fencing material you can imagine.
0:14:56 > 0:14:58So for two months a year,
0:14:58 > 0:15:03the normally peaceful Estes Park fills with chaos and disruption.
0:15:05 > 0:15:09Believe it or not, we're not exempt from this sort of behaviour
0:15:09 > 0:15:12right here in the UK. Take a look at this poor chap,
0:15:12 > 0:15:15given a right run for his money in a London park.
0:15:18 > 0:15:19Dear, oh, dear.
0:15:19 > 0:15:22You really wouldn't want to get hit by one of these things.
0:15:22 > 0:15:24They are tremendously powerful animals.
0:15:27 > 0:15:30Enormous rampant deer are one thing,
0:15:30 > 0:15:32but when it comes to disruption,
0:15:32 > 0:15:36sometimes it's the little things that pack the biggest punch.
0:15:36 > 0:15:39Our trail of extraordinary animal invasions
0:15:39 > 0:15:42now leads us to Christmas Island in the Indian Ocean.
0:15:43 > 0:15:47The island looks like the perfect tropical paradise.
0:15:49 > 0:15:53# How would you like to spend Christmas
0:15:53 > 0:15:57# On Christmas Island? #
0:15:57 > 0:16:00But there's one Christmas when you really wouldn't have wanted
0:16:00 > 0:16:02to be on Christmas Island.
0:16:11 > 0:16:13The beach turned red,
0:16:13 > 0:16:17as billions of tiny crabs marched out of the sea
0:16:17 > 0:16:19on an unstoppable journey inland.
0:16:22 > 0:16:24The entire village was engulfed.
0:16:28 > 0:16:32And the onslaught lasted for three solid weeks.
0:16:43 > 0:16:48So why had so many miniature crabs swamped the village?
0:16:50 > 0:16:53Well, exactly two months earlier,
0:16:53 > 0:16:55the islanders had had to deal with this...
0:17:00 > 0:17:04..an invasion of much bigger crabs heading in the opposite direction,
0:17:04 > 0:17:06towards the beach.
0:17:08 > 0:17:11These are a species called Christmas Island crabs
0:17:11 > 0:17:13and they're on their annual migration
0:17:13 > 0:17:16from their home in the rainforest to the water's edge.
0:17:19 > 0:17:20They may be land crabs,
0:17:20 > 0:17:23but they still need to lay their eggs in the sea.
0:17:27 > 0:17:30So once a year, the arrival of the monsoon rains
0:17:30 > 0:17:34prompts the crabs to emerge from all over the forest.
0:17:36 > 0:17:40Crab expert Dr Simon Webster has been studying
0:17:40 > 0:17:42this perfectly timed march to the water's edge.
0:17:44 > 0:17:47They're tiny animals. They're only 20 centimetres across
0:17:47 > 0:17:50and they can travel 300 metres an hour.
0:17:50 > 0:17:53They may travel anywhere between nine to 15 kilometres,
0:17:53 > 0:17:55which is an enormous distance for a crab.
0:17:55 > 0:17:58It's the equivalent of marathon distance.
0:17:59 > 0:18:05They must migrate, mate and spawn within one lunar cycle,
0:18:05 > 0:18:09so within 28 days, they must complete everything.
0:18:09 > 0:18:12And it's a race against time. Out here in the open,
0:18:12 > 0:18:16the crabs' biggest threat to survival is drying out.
0:18:19 > 0:18:21If the rains stop during the migration,
0:18:21 > 0:18:26the migration fails, and tens of thousands of crabs will die.
0:18:26 > 0:18:31They will stop to drink on the way any available standing water,
0:18:31 > 0:18:35even liquid mud. Should the sun come out,
0:18:35 > 0:18:37even though the humidity stays very high,
0:18:37 > 0:18:38the crabs will dry out.
0:18:40 > 0:18:42As they emerge from the rainforest,
0:18:42 > 0:18:45they instinctively know which way to go.
0:18:48 > 0:18:50It's the same well-trodden path
0:18:50 > 0:18:53that their ancestors have been taking for generations.
0:18:59 > 0:19:02It's a route that takes them straight through the village,
0:19:02 > 0:19:07and headlong into the everyday lives of the long-suffering humans here.
0:19:09 > 0:19:12Roads are carpeted with red
0:19:12 > 0:19:16and millions upon millions of these crabs come out of the rainforest,
0:19:16 > 0:19:19cross roads, go through people's houses, down to the sea.
0:19:19 > 0:19:23It is one of the most spectacular animal migrations on earth.
0:19:23 > 0:19:25These crabs are very good climbers.
0:19:25 > 0:19:29They can climb up the corner of a room quite easily.
0:19:31 > 0:19:34And they will go through any doorway, wardrobe,
0:19:34 > 0:19:38they will end up in drawers, in sinks, anywhere.
0:19:41 > 0:19:44Having run the gauntlet of all of the human obstacles,
0:19:44 > 0:19:46the crabs arrive at the beach, exhausted.
0:19:47 > 0:19:51But there's only time for a quick dip to refresh their parched bodies
0:19:51 > 0:19:54before their thoughts turn quickly to mating
0:19:54 > 0:19:57and releasing their eggs into the sea.
0:19:59 > 0:20:03Once they're set adrift, the crab larvae are completely dependent
0:20:03 > 0:20:05on the movements of the tides.
0:20:07 > 0:20:09And it's a dangerous world.
0:20:10 > 0:20:13Most years these babies end up as fish food,
0:20:13 > 0:20:16or get swept out and lost in deep waters.
0:20:16 > 0:20:20But once, maybe twice a decade, they get lucky.
0:20:20 > 0:20:23The tide brings vast numbers of tiny crabs back to the shore
0:20:23 > 0:20:26to begin their march into the forest.
0:20:30 > 0:20:33The small crabs, when they emerge, are just a few millimetres across,
0:20:33 > 0:20:38about half the size of a small fingernail.
0:20:38 > 0:20:41They're almost transparent. You can see the organs within them.
0:20:41 > 0:20:43They are very, very delicate.
0:20:43 > 0:20:48They have the same sort of texture as a pea.
0:20:48 > 0:20:50So they're very easy to crush.
0:20:50 > 0:20:55The islanders take great care not to kill any animals at all.
0:20:55 > 0:20:58There are underpasses on the roads, roads are closed,
0:20:58 > 0:21:00there are even a couple of crab bridges.
0:21:00 > 0:21:05It's hard to imagine how something so fragile can survive the journey.
0:21:08 > 0:21:12But having completed this amazing migration back to the shore,
0:21:12 > 0:21:15what happens next is actually a bit of a mystery.
0:21:15 > 0:21:19Just a month after the first tiny crabs arrive on the beach,
0:21:19 > 0:21:22they all disappear back into the rainforest -
0:21:22 > 0:21:24millions and millions of them,
0:21:24 > 0:21:29all going their own separate ways and living in isolation.
0:21:29 > 0:21:33What all these stories show is that when it comes to reproduction,
0:21:33 > 0:21:35nature is a force to be reckoned with.
0:21:35 > 0:21:39Whether it's a plague of insects looking for love,
0:21:39 > 0:21:43a mob of amorous elk scaring off the competition,
0:21:43 > 0:21:48or an invasion of crabs caused by a race to mate...
0:21:49 > 0:21:52..when we get in the way of nature's need to breed,
0:21:52 > 0:21:55it can stop us in our tracks.
0:21:58 > 0:22:00For our next group of show-stopping events,
0:22:00 > 0:22:02we see what happens when we humans
0:22:02 > 0:22:05accidentally tip the balance of nature.
0:22:05 > 0:22:08From a fearsome army of insect invaders
0:22:08 > 0:22:12to an island paradise forced into an eerie silence.
0:22:12 > 0:22:16And the escaped pets that have got out of control,
0:22:16 > 0:22:19with terrifying consequences.
0:22:19 > 0:22:24But we start in Africa, home to red-billed Quelea -
0:22:24 > 0:22:28a delicate little bird no bigger than a sparrow.
0:22:28 > 0:22:31A member of the artistic weaver bird family,
0:22:31 > 0:22:33it showcases its talents
0:22:33 > 0:22:36by making an intricate grass nest to attract a mate.
0:22:37 > 0:22:40So why would this cute and harmless little bird
0:22:40 > 0:22:45be the most feared and hated bird in Africa?
0:22:46 > 0:22:48Because it does this.
0:22:56 > 0:23:01Huge flocks of Quelea, up to two million strong,
0:23:01 > 0:23:05swoop across the landscape turning the sky black.
0:23:05 > 0:23:09A flurry of wings all beating furiously.
0:23:23 > 0:23:26And when they land to refuel, these ravenous swarms
0:23:26 > 0:23:30can get through almost 20 tonnes of grain a day.
0:23:31 > 0:23:35Enough to wipe out an entire farm
0:23:35 > 0:23:40and cause £40 million worth of damage every year.
0:23:42 > 0:23:45They're now one of the world's biggest agricultural pests,
0:23:45 > 0:23:48and their devastating appetite has given them
0:23:48 > 0:23:50the nickname "feathered locust".
0:23:55 > 0:23:59But how do these super-flocks of Quelea get so big?
0:24:02 > 0:24:05A single bird may not look like much of a threat,
0:24:05 > 0:24:08but it's their habit of hanging around in huge gangs
0:24:08 > 0:24:10that's made them a menace.
0:24:10 > 0:24:12With a population of 1.5 billion,
0:24:12 > 0:24:16Quelea are now the world's most abundant wild bird.
0:24:18 > 0:24:21Not bad for a species that's only found in Africa.
0:24:24 > 0:24:27Professor Robert Cheke has been trying to find
0:24:27 > 0:24:29the key to the Quelea's success.
0:24:30 > 0:24:34The breeding process of Quelea birds is really quite spectacular,
0:24:34 > 0:24:37and very unusual, because they breed in huge colonies,
0:24:37 > 0:24:41and a remarkable thing about the breeding of Quelea birds
0:24:41 > 0:24:44is that each female will lay usually about three eggs.
0:24:44 > 0:24:47They don't just lay three eggs once a year,
0:24:47 > 0:24:50they may lay three eggs three times a year,
0:24:50 > 0:24:54or in very rare occasions, up to four or five times a year.
0:24:54 > 0:24:57So we're looking at maybe ten chicks per female.
0:24:59 > 0:25:03So when you do the maths, that's ten times a million breeding pairs -
0:25:03 > 0:25:08potentially ten million new birds a year in just one flock.
0:25:08 > 0:25:10But that can't be the whole story,
0:25:10 > 0:25:13because not all of these chicks will survive,
0:25:13 > 0:25:15and having lots of young is a strategy
0:25:15 > 0:25:18that many other species of birds use.
0:25:18 > 0:25:22So there must be something else that's helping them thrive.
0:25:24 > 0:25:28Experts believe it's connected to their wandering ways.
0:25:28 > 0:25:32You see, something that sets them apart from many other birds
0:25:32 > 0:25:35is that they are completely nomadic.
0:25:35 > 0:25:39So with no permanent home, they simply go where the food is.
0:25:39 > 0:25:43Flocks cover tens of thousands of kilometres a year
0:25:43 > 0:25:46searching for the best places to feed and nest.
0:25:46 > 0:25:49That means following the rain.
0:25:51 > 0:25:52The grains that the Quelea feed on
0:25:52 > 0:25:55can only germinate after a good soaking,
0:25:55 > 0:25:58and rain also helps flush out insects for the chicks to eat.
0:26:00 > 0:26:03Downpours in Africa can be few and far between,
0:26:03 > 0:26:05but Quelea are experts
0:26:05 > 0:26:07at being in the right place at the right time.
0:26:10 > 0:26:13Somehow they seem to know where the rain has fallen.
0:26:13 > 0:26:16It's possible that they can detect this by physically seeing
0:26:16 > 0:26:19clouds and rain formations in the distance,
0:26:19 > 0:26:21or they may be able to detect relative humidity
0:26:21 > 0:26:23and moisture in the air.
0:26:23 > 0:26:25But however they do it,
0:26:25 > 0:26:29the Quelea are always in the perfect place to feed.
0:26:34 > 0:26:37In the morning, the birds leave en masse,
0:26:37 > 0:26:40and it's thought that perhaps some particular birds
0:26:40 > 0:26:43which have been very successful the night before in finding food
0:26:43 > 0:26:46will act as scouts and guide the other birds,
0:26:46 > 0:26:51which are perhaps less efficiently capable of finding their own food,
0:26:51 > 0:26:52to particular sites.
0:26:53 > 0:26:56It's a winning strategy for the birds -
0:26:56 > 0:26:59and the losers are the poor farmers.
0:27:00 > 0:27:03Ironically, though, part of the responsibility
0:27:03 > 0:27:07for the devastation they cause actually lies with man.
0:27:08 > 0:27:11As Africa's farmland continues to expand,
0:27:11 > 0:27:15it's providing these birds with fields full of easy pickings,
0:27:15 > 0:27:20helping them reach plague-like proportions.
0:27:20 > 0:27:24Facing starvation, farmers desperately try to tackle the problem.
0:27:24 > 0:27:28But despite killing millions of Quelea every year,
0:27:28 > 0:27:30the population bounces right back.
0:27:32 > 0:27:34You have to really admire the red-billed Quelea bird,
0:27:34 > 0:27:38because it is such a successful and exciting bird.
0:27:40 > 0:27:43They seem to have an uncanny ability to survive
0:27:43 > 0:27:45whatever humans throw at them.
0:27:45 > 0:27:47And despite their destructive side,
0:27:47 > 0:27:50there's a real beauty to these birds.
0:27:53 > 0:27:57The way those huge flocks of birds so effortlessly change shape
0:27:57 > 0:28:00and direction is absolutely spectacular.
0:28:00 > 0:28:03And, you know, in the UK, we have a couple of species
0:28:03 > 0:28:06that perform similarly aerobatic manoeuvres -
0:28:06 > 0:28:08the knot and the starling.
0:28:08 > 0:28:10And scientists have always wanted to know
0:28:10 > 0:28:15how they coordinate these displays, or murmurations, as we call them.
0:28:15 > 0:28:18The latest thinking on how they do it
0:28:18 > 0:28:21is that each bird follows its seven closest neighbours,
0:28:21 > 0:28:23not just the bird in front.
0:28:24 > 0:28:28And it's not so much the distance between them that's important,
0:28:28 > 0:28:30but the direction they're travelling in.
0:28:31 > 0:28:34By simply sticking to these key rules,
0:28:34 > 0:28:37all the birds stay in perfect formation,
0:28:37 > 0:28:39whether it's a few thousand knot...
0:28:41 > 0:28:43..or a few million Quelea.
0:28:47 > 0:28:49The impact that Quelea have on us humans
0:28:49 > 0:28:52is partly thanks to the perfect opportunity
0:28:52 > 0:28:54that we've given them to expand.
0:28:54 > 0:28:58But sometimes we've not just created the right conditions
0:28:58 > 0:29:02for an invasion, we've actually helped launch the attack.
0:29:04 > 0:29:07We head to the southern United States
0:29:07 > 0:29:10on the trail of a tiny but fearless army...
0:29:13 > 0:29:16..the South American fire ant.
0:29:18 > 0:29:22Voracious predators acting as one coordinated unit,
0:29:22 > 0:29:24the ants attack en masse...
0:29:27 > 0:29:31..capable of taking down prey many times their own size.
0:29:33 > 0:29:35And fuelling their advance,
0:29:35 > 0:29:39the queen is able to produce up to 2,000 eggs a day.
0:29:44 > 0:29:49Not even floodwater and rivers pose any real barrier.
0:29:51 > 0:29:55By interlocking their bodies to create a floating structure,
0:29:55 > 0:29:58they can transport the whole colony to safety,
0:29:58 > 0:30:01queen, eggs, larvae and all.
0:30:05 > 0:30:08Having escaped their native home,
0:30:08 > 0:30:12they're now busy setting up colonies all over the globe.
0:30:12 > 0:30:16Arriving in the US on a cargo ship in the 1930s
0:30:16 > 0:30:20they've become an unstoppable ecological disaster
0:30:20 > 0:30:25attacking anything in their path.
0:30:25 > 0:30:29Unlike many other ants they have a very special sting.
0:30:29 > 0:30:33Using their jaws to grab hold, the other end of their body whips
0:30:33 > 0:30:38round to inject their victim with a venom that burns like fire.
0:30:38 > 0:30:42But what's worse is the ant now signals to all of the others
0:30:42 > 0:30:45that an attack is under way and, within minutes,
0:30:45 > 0:30:49a whole army of them show up - all joining in the fight.
0:30:49 > 0:30:52It's not long before you're left with a swollen limb
0:30:52 > 0:30:55full of angry stings.
0:30:55 > 0:30:59Unstoppable, you might think, but as they hit North America's
0:30:59 > 0:31:05urban landscape, a very weird Achilles heel was unveiled -
0:31:05 > 0:31:08a death wish for electrocution.
0:31:11 > 0:31:13You see, they seem to love nothing more
0:31:13 > 0:31:17than piling into electrical circuits in their thousands
0:31:17 > 0:31:21to meet a crackly fate,
0:31:21 > 0:31:25one after the other until the entire electrical system blows.
0:31:30 > 0:31:33So why on earth would a seemingly indestructible creature
0:31:33 > 0:31:36want to commit mass suicide?
0:31:40 > 0:31:44One theory is that the ants are attracted by the magnetic field
0:31:44 > 0:31:49surrounding electrical equipment.
0:31:49 > 0:31:53Convincing on the face of it, but proof was elusive.
0:31:53 > 0:31:58Entomologist and fire ant expert Dr Karen Vail
0:31:58 > 0:32:00has been investigating the ants
0:32:00 > 0:32:03and their fatal attraction to electricity.
0:32:03 > 0:32:05Fire ants spend most time under the ground.
0:32:05 > 0:32:09So they don't communicate so much visually as they do through using chemicals.
0:32:09 > 0:32:11They use things like alarm pheromones
0:32:11 > 0:32:15to indicate a predator's nearby and to recruit other ants
0:32:15 > 0:32:17to come and defend the colony.
0:32:17 > 0:32:21What this means is that they can respond to danger en masse.
0:32:21 > 0:32:27It just takes one ant to signal for the whole colony to respond.
0:32:27 > 0:32:30Most people don't realise that they're stepping in a mound
0:32:30 > 0:32:33so they have hundreds of ants running up their legs at a time.
0:32:33 > 0:32:36That one will bite, release an alarm pheromone,
0:32:36 > 0:32:38which causes the others to bite and sting.
0:32:38 > 0:32:43It's pretty amazing how organised 240,000 workers can be.
0:32:43 > 0:32:45But this doesn't explain why they're drawn
0:32:45 > 0:32:47to lethal electrical fields.
0:32:49 > 0:32:51We used to think they were attracted to magnetic fields
0:32:51 > 0:32:55but what we think happens is that the fire ants are out scouting,
0:32:55 > 0:33:00looking for food, and they come across an electrical current
0:33:00 > 0:33:03and they get killed.
0:33:03 > 0:33:05And they release an alarm pheromone
0:33:05 > 0:33:07and when they release the alarm pheromone,
0:33:07 > 0:33:09other workers are attracted to that.
0:33:09 > 0:33:12They come in, they get killed by the electrical current,
0:33:12 > 0:33:15causes more alarm pheromone to be released and before you know it
0:33:15 > 0:33:18the majority of the colony has responded to this alarm pheromone
0:33:18 > 0:33:21and they end up shorting out equipment.
0:33:23 > 0:33:26So rather than electricity drawing the ants in,
0:33:26 > 0:33:30it's actually the ants themselves responding to calls of help
0:33:30 > 0:33:34from their accidentally frazzled comrades.
0:33:34 > 0:33:38We estimate about 6 billion are spent a year
0:33:38 > 0:33:42in paying for medical costs, controlling the ants,
0:33:42 > 0:33:46replacing livestock, equipment and other damage that they cause.
0:33:48 > 0:33:526 billion! That's an incredible amount of money!
0:33:53 > 0:33:58But scientists were at a loss as to how they could tackle the problem.
0:33:58 > 0:34:01Electricity was never going to have any meaningful effect
0:34:01 > 0:34:02on their numbers.
0:34:04 > 0:34:07Then they discovered something a lot more effective
0:34:07 > 0:34:10and much, much weirder.
0:34:13 > 0:34:16The ants had an even bigger nemesis.
0:34:17 > 0:34:21In an incredible twist, the very same pheromone that leads them
0:34:21 > 0:34:27to their death in electrical systems also attracts their arch enemy -
0:34:27 > 0:34:31a tiny South American fly,
0:34:31 > 0:34:35a fraction of the size of the ants themselves.
0:34:35 > 0:34:38Found in the ants' original homeland in South America,
0:34:38 > 0:34:43the mere presence of this predator can stop them leaving their nests.
0:34:43 > 0:34:46The fly is attracted to the alarm pheromones.
0:34:46 > 0:34:49It uses them to home in on the ant before injecting
0:34:49 > 0:34:50an egg into their body.
0:34:51 > 0:34:58The larvae then hatches and grows, feeding on the ant's brain.
0:34:58 > 0:35:01When it's ready it secretes an enzyme that dissolves
0:35:01 > 0:35:05the connection between the head and body until...
0:35:05 > 0:35:07the head pops off.
0:35:14 > 0:35:17What a wonderfully weird way to go.
0:35:17 > 0:35:21So sci-fi, straight out of the movies.
0:35:21 > 0:35:24In the end though, it's the ants' key strength -
0:35:24 > 0:35:28their ability to act as one, in synchronicity,
0:35:28 > 0:35:31all controlled by their smells -
0:35:31 > 0:35:33which is leading to their demise.
0:35:33 > 0:35:38You see, the flies are quite literally sniffing them out.
0:35:41 > 0:35:45Next, our trail of nature's weirdest invaders takes us
0:35:45 > 0:35:47to an island paradise.
0:35:49 > 0:35:52The tiny island of Guam in the western Pacific.
0:35:54 > 0:35:57Once a haven of island biodiversity,
0:35:57 > 0:36:00rich in native species of birds and reptiles.
0:36:02 > 0:36:06Its wildlife, having evolved in the absence of wild predators
0:36:06 > 0:36:07had little to fear...
0:36:09 > 0:36:15..until the 1950s when species started to systematically disappear.
0:36:15 > 0:36:18The native bird species simply vanished
0:36:18 > 0:36:23and the once raucous rainforest fell completely silent.
0:36:28 > 0:36:31What happened next was truly bizarre.
0:36:31 > 0:36:37The silent forest filled with spiders,
0:36:37 > 0:36:43their webs continually expanding and covering every available space.
0:36:46 > 0:36:53So what could have stunned an island paradise into sinister silence?
0:36:55 > 0:36:58Well, to answer this we need a quick history lesson about Guam.
0:37:04 > 0:37:07Occupying a strategic position in the Western Pacific,
0:37:07 > 0:37:11the island was used as a US military base during the Second World War.
0:37:14 > 0:37:18But on one of the military machines left behind was a visitor
0:37:18 > 0:37:22that would change the face of Guam for ever -
0:37:26 > 0:37:29..the Papua New Guinean brown tree snake.
0:37:32 > 0:37:38Used to a competitive world, the snake found itself on an island full of food.
0:37:38 > 0:37:42And having never seen a predator, the native fauna simply
0:37:42 > 0:37:46didn't know how to respond, they were easy pickings.
0:37:50 > 0:37:54The snake gradually spread out across the island
0:37:54 > 0:37:57decimating native bird populations as it went.
0:37:58 > 0:38:02But one species' loss was another species' gain.
0:38:02 > 0:38:05With fewer birds to keep them in check
0:38:05 > 0:38:08the spider population exploded.
0:38:09 > 0:38:14Guam now has 40 times more spiders than its neighbouring islands.
0:38:18 > 0:38:20But even the disappearance of their food source
0:38:20 > 0:38:23didn't stop their population explosion.
0:38:23 > 0:38:28Unusually for snakes the brown tree snake is not a fussy eater.
0:38:28 > 0:38:30They're happy to scavenge too.
0:38:32 > 0:38:36As a result, their numbers continued to skyrocket.
0:38:39 > 0:38:40In the face of the onslaught
0:38:40 > 0:38:45the US government tried anything they could think of.
0:38:45 > 0:38:49Trapping, searching for the snakes by sight and with dogs.
0:38:51 > 0:38:55But confronted with certain defeat they've now decided to deploy
0:38:55 > 0:38:59a radical tactic and engage a very covert
0:38:59 > 0:39:02and specialist parachute regiment...of mice.
0:39:04 > 0:39:08Yes, you heard correctly, lacing dead mice with a chemical
0:39:08 > 0:39:12found in paracetamol that is poisonous to the snakes.
0:39:12 > 0:39:16They've started to drop them from the air into the forest.
0:39:17 > 0:39:19And the snakes' relaxed eating habits means
0:39:19 > 0:39:22they readily take the bait.
0:39:23 > 0:39:27But as effective as this might be, it's thought it will merely
0:39:27 > 0:39:32control the population rather than eradicate the snakes completely.
0:39:32 > 0:39:36An accidental invader that's really overstayed its welcome.
0:39:39 > 0:39:44But then, not all invasions have been started accidentally.
0:39:44 > 0:39:49This brute over here is a cane toad, a resident of South America.
0:39:49 > 0:39:52But it was introduced to Australia to try and control the voracious appetite
0:39:52 > 0:39:55of the sugar cane beetle.
0:39:55 > 0:39:58The trouble is it reneged on its side of the bargain,
0:39:58 > 0:40:01and rather than eating the beetles, it's been gobbling up
0:40:01 > 0:40:03the native wildlife ever since.
0:40:03 > 0:40:07It's a classic case of biological control gone bad.
0:40:09 > 0:40:11Other species have been introduced as food,
0:40:11 > 0:40:14the most notable example perhaps being this -
0:40:14 > 0:40:17the common or garden snail.
0:40:17 > 0:40:20This was brought to the UK by the Romans.
0:40:20 > 0:40:22We no longer have a taste for these animals
0:40:22 > 0:40:24but they do have a taste for things that we grow.
0:40:24 > 0:40:26And given that one of them
0:40:26 > 0:40:30can produce 480 more in the space of a year they've pretty much
0:40:30 > 0:40:34turned themselves into the scourge of the British gardener.
0:40:34 > 0:40:37But our next weird invasion is altogether more dangerous
0:40:37 > 0:40:42and ruthless than the average snail, and a whole lot more intimidating.
0:40:42 > 0:40:43- Got it?- You all right?
0:40:43 > 0:40:45Yeah, go, go, go, go.
0:40:47 > 0:40:53In August 2012 the largest Burmese python ever recorded was captured.
0:40:53 > 0:40:57Weighing in at just under 75kg, it took four men to wrestle it
0:40:57 > 0:40:59out of the bushes.
0:40:59 > 0:41:04But this massive Burmese python is nowhere near Burma.
0:41:04 > 0:41:08In fact, it's being dragged out of the Florida Everglades.
0:41:10 > 0:41:13And worryingly, it's not the only one.
0:41:13 > 0:41:15Argh!
0:41:15 > 0:41:18Just like on Guam, the local wildlife had never seen
0:41:18 > 0:41:22anything like it before, and scientists have recorded
0:41:22 > 0:41:27a massive drop of up to 99% in some species of local mammals.
0:41:27 > 0:41:33Possums, raccoons and bobcats have all been badly hit.
0:41:33 > 0:41:36And even the Everglades' top predator, the alligator,
0:41:36 > 0:41:38appears to have met its match.
0:41:38 > 0:41:42Clashes between alligators and pythons are common.
0:42:01 > 0:42:06So what is a giant snake from South-East Asia doing over 14,000km
0:42:06 > 0:42:09away from home?
0:42:10 > 0:42:15Well, it's all down to our desire for exotic pets.
0:42:15 > 0:42:18Burmese pythons are bought as small and inoffensive snakes
0:42:18 > 0:42:23at a manageable length, but this doesn't last long.
0:42:25 > 0:42:29The snakes all too often outgrow their owners' ability to keep them.
0:42:31 > 0:42:37This gigantic python, with a staggering 76cm girth,
0:42:37 > 0:42:40made the headlines when it was removed from one of Florida's
0:42:40 > 0:42:42residential neighbourhoods.
0:42:42 > 0:42:44Quite capable of eating an adult human,
0:42:44 > 0:42:47it was dangerously out of control.
0:42:48 > 0:42:52And faced with overgrown snakes many other owners have been known
0:42:52 > 0:42:53to set their animals free.
0:42:53 > 0:42:56Others simply break out of their cages.
0:42:59 > 0:43:04The trouble is that Florida turns out to be their ideal habitat -
0:43:04 > 0:43:07plenty of food, water and cover.
0:43:07 > 0:43:10And unlike their home in Burma where they're hunted for their skin
0:43:10 > 0:43:15and their habitat is in decline, life here is pretty good.
0:43:17 > 0:43:20The local wildlife doesn't stand a chance.
0:43:21 > 0:43:24Pythons kill their prey by constriction -
0:43:24 > 0:43:28squeezing the life out of it and then swallowing it whole.
0:43:29 > 0:43:32Like all snakes, they have an incredibly flexible jaw,
0:43:32 > 0:43:37enabling them to swallow prey many times the width of their own body.
0:43:41 > 0:43:44And as if being able to swallow bigger prey than yourself
0:43:44 > 0:43:48wasn't weird enough, after a meal of this size,
0:43:48 > 0:43:51the snake won't need to eat again for months.
0:43:55 > 0:43:58All together, it's a recipe for success
0:43:58 > 0:44:00and the authorities are now removing pythons
0:44:00 > 0:44:03from the Everglades in their hundreds.
0:44:09 > 0:44:13These weird events show us what happens when man accidentally
0:44:13 > 0:44:18interferes with nature's balance and it unleashes its power,
0:44:18 > 0:44:21whether it's locust-like plagues of birds,
0:44:21 > 0:44:25fearless insects on a global crusade,
0:44:25 > 0:44:29or snakes that develop a taste for the local cuisine.
0:44:29 > 0:44:34Given the right conditions nature can leave us feeling both powerless
0:44:34 > 0:44:35and awestruck.
0:44:37 > 0:44:42In this next section we move from the devastating power
0:44:42 > 0:44:47of biological onslaughts to atmospheric ones
0:44:47 > 0:44:52and to some of the most spectacular meteorological mysteries on the planet.
0:44:52 > 0:44:55From apocalyptic clouds...
0:44:55 > 0:44:57That is one crazy-looking storm.
0:44:59 > 0:45:02..to lightning that's lost its storm.
0:45:05 > 0:45:08And for that story, we travel to Iceland.
0:45:10 > 0:45:15April 2010, a volcano that had been dormant for 200 years
0:45:15 > 0:45:17suddenly erupted.
0:45:20 > 0:45:23This is as close as we dare go to this huge plume.
0:45:25 > 0:45:30As lava oozed out down the mountain, huge plumes of ash
0:45:30 > 0:45:33were sent skywards, reaching heights of over 10km.
0:45:34 > 0:45:37But it's when the ash cloud started spreading out
0:45:37 > 0:45:41across Northern Europe that the eruption really hit the headlines.
0:45:43 > 0:45:45UK airspace is closed for the first time.
0:45:45 > 0:45:48All flights have been grounded amid safety fears
0:45:48 > 0:45:52as a cloud of volcanic ash drifts over Britain.
0:45:52 > 0:45:56And with all eyes trained on that ash cloud people started to notice
0:45:56 > 0:45:58freak flashes of light.
0:45:58 > 0:46:01Whoa! Look at the light.
0:46:04 > 0:46:07There wasn't a storm cloud in sight
0:46:07 > 0:46:10but bolts of lightning were coming in thick and fast.
0:46:15 > 0:46:18So why on earth was a volcano alive with lightning?
0:46:22 > 0:46:26It turns out this strange spectacle isn't as rare as you might think.
0:46:28 > 0:46:32First spotted way back in 79AD, it's been making the occasional
0:46:32 > 0:46:35mystifying appearance ever since.
0:46:36 > 0:46:38But to work out what causes it,
0:46:38 > 0:46:41first we need to get to grips with lightning.
0:46:41 > 0:46:43LIGHTNING CRACKS
0:46:49 > 0:46:52Not easy, when even Graeme Anderson,
0:46:52 > 0:46:54one of the top brains at the Met Office,
0:46:54 > 0:46:56admits it's a tricky subject.
0:46:56 > 0:46:59The way that lightning's generated in normal thunderstorms
0:46:59 > 0:47:03isn't completely understood, and lightning from volcanic eruptions
0:47:03 > 0:47:05is even rarer and even more treacherous
0:47:05 > 0:47:08if you want to get in and try and study it.
0:47:08 > 0:47:12Most lightning's created within shower clouds and thunderclouds.
0:47:12 > 0:47:17Those clouds are created when the atmosphere is said to be unstable.
0:47:17 > 0:47:21In these stormy conditions moisture in the air is drawn high
0:47:21 > 0:47:23up into the clouds.
0:47:23 > 0:47:29Here it reaches temperatures so cold that the water droplets turn to ice.
0:47:29 > 0:47:32And this is where the physics gets weird -
0:47:32 > 0:47:35because to get lightning, which has a temperature hotter than
0:47:35 > 0:47:40the surface of the sun, you actually need large quantities of ice.
0:47:41 > 0:47:44Those ice particles are bouncing and rubbing off each other.
0:47:44 > 0:47:47And over time, this leads to a generation of charge
0:47:47 > 0:47:49that spreads out within the cloud
0:47:49 > 0:47:52and when the charge within the cloud becomes big enough
0:47:52 > 0:47:54it can lead to a spark.
0:47:54 > 0:47:57That's what you see as lightning, is all of that charge
0:47:57 > 0:48:00rushing down a very narrow channel.
0:48:03 > 0:48:05It becomes very, very hot,
0:48:05 > 0:48:07much hotter than the surface of the sun,
0:48:07 > 0:48:11and the air actually explodes which creates the flash of light
0:48:11 > 0:48:12and rumble of thunder that you hear.
0:48:12 > 0:48:17So it's ice that causes the spark to set off the lightning,
0:48:17 > 0:48:20but how on earth can this happen in a volcano where temperatures
0:48:20 > 0:48:23can reach over 2,000 degrees Fahrenheit?
0:48:23 > 0:48:27Look at the debris there. Large pieces of rock and lightning.
0:48:27 > 0:48:29So much kinetic energy being released.
0:48:29 > 0:48:31Stay here. Can you stay?
0:48:31 > 0:48:34- I can't, the wind is just too strong.- OK.
0:48:34 > 0:48:38Well, one theory that attempts to explain it
0:48:38 > 0:48:41is the Dirty Thunderstorm Hypothesis.
0:48:41 > 0:48:44The lightning within a volcanic ash plume is generated
0:48:44 > 0:48:47from moisture emitted from the volcano,
0:48:47 > 0:48:51rising up into the atmosphere and condensing into water droplets
0:48:51 > 0:48:56that then carry on upwards and freeze, creating ice particles.
0:48:56 > 0:48:58You need a particularly vigorous eruption -
0:48:58 > 0:49:02one that's going to really throw out a lot of heat and energy
0:49:02 > 0:49:04and really pump a lot of moisture into the upper atmosphere,
0:49:04 > 0:49:10several kilometres up into the air so that it reaches those cold levels of the atmosphere.
0:49:10 > 0:49:14Not all volcanoes will have enough energy to get water high enough
0:49:14 > 0:49:19to freeze - which is what makes volcanic lightning so unpredictable.
0:49:19 > 0:49:24But as long as you have ice, you have your key ingredient for lightning.
0:49:26 > 0:49:28That process of generating lightning, in the same way
0:49:28 > 0:49:32as in a thunderstorm, can work within a volcanic ash plume.
0:49:32 > 0:49:36Regions of charge within the plume will lead to a spark,
0:49:36 > 0:49:39which is the lightning strike that you can see.
0:49:39 > 0:49:43This is catastrophic. Whoa, look at the lightning there.
0:49:48 > 0:49:50Dramatic stuff!
0:49:50 > 0:49:55But did you know that what we perceive as a single bolt of lightning
0:49:55 > 0:50:00could be composed of up to as many as 25 super-fast sub-flashes?
0:50:00 > 0:50:05And they're so quick that we still see it as a single bolt.
0:50:05 > 0:50:09And they're not only fast, they're also incredibly intense
0:50:09 > 0:50:11and incredibly bright.
0:50:11 > 0:50:14So much so that they burn an image onto our eyes
0:50:14 > 0:50:17that lasts for several seconds
0:50:17 > 0:50:20despite the fact that their combined duration
0:50:20 > 0:50:24is less than 100 millionths of one second!
0:50:26 > 0:50:29But weather doesn't always have to be loud and flashy
0:50:29 > 0:50:32to have us stopped in our tracks.
0:50:32 > 0:50:36Sometimes even a silky sky presents us
0:50:36 > 0:50:40with events that are more science fiction than science.
0:50:40 > 0:50:44In Burketown, Australia, every September,
0:50:44 > 0:50:49residents wake up to these incredible scenes.
0:50:49 > 0:50:54It's an amazing cloud formation here in Burketown. Wow!
0:50:56 > 0:51:00Row after row of long tube-like clouds
0:51:00 > 0:51:03stretching from horizon to horizon.
0:51:04 > 0:51:08The phenomenon had been dubbed the Morning Glory by the locals.
0:51:11 > 0:51:14And every year as spring arrives so do the clouds.
0:51:18 > 0:51:21So what brings this Morning Glory to Burketown?
0:51:23 > 0:51:27Gavin Pretor-Pinney, a dedicated cloud spotter,
0:51:27 > 0:51:29has spent a lifetime staring skywards.
0:51:31 > 0:51:35The clouds are expressions on the face of the atmosphere
0:51:35 > 0:51:40and they can be read like the expressions on the face
0:51:40 > 0:51:44of a person. They reveal the moods of the atmosphere.
0:51:46 > 0:51:49When it arrives this cloud looks very dramatic,
0:51:49 > 0:51:54the sky is clear, you see this tube rolling along towards you,
0:51:54 > 0:51:59and then as it passes over, momentarily the sky becomes overcast
0:51:59 > 0:52:03and then as it moves on and you are left in its wake,
0:52:03 > 0:52:08the sky clears again and with that movement, you get the rushing winds
0:52:08 > 0:52:13as it approaches and then once it's over you, the wind momentarily drops
0:52:13 > 0:52:16and then as it passes on, the wind picks up again.
0:52:16 > 0:52:21So it's quite an experience when one of these Morning Glory clouds passes over you.
0:52:21 > 0:52:25OK, if clouds are trying to tell us something about our skies,
0:52:25 > 0:52:28what on earth is the Morning Glory saying to us?
0:52:28 > 0:52:32The peninsula gets heated up by the sun during the day.
0:52:32 > 0:52:36The sea breezes come in both sides, collide, and set off this wave
0:52:36 > 0:52:41which travels through the night, arriving at Burketown,
0:52:41 > 0:52:46and within that wave of air, a roll of cloud can form.
0:52:48 > 0:52:53And although no other place puts on quite such a spectacular show as Burketown,
0:52:53 > 0:52:56you do get the odd, show-stopping,
0:52:56 > 0:52:59single roll cloud appearing in other coastal areas.
0:52:59 > 0:53:02What is it? It's right above us!
0:53:03 > 0:53:07And these aren't the only clouds that have had us mystified.
0:53:07 > 0:53:11Across the globe people have rushed out to record
0:53:11 > 0:53:13their own seemingly unexplainable skies.
0:53:16 > 0:53:20It's a very strange-looking cloud formation.
0:53:20 > 0:53:24But when it comes to causing alarm, few clouds can compete
0:53:24 > 0:53:27with the cumulonimbus.
0:53:27 > 0:53:31It looks a little bit like an atom bomb cloud, a mushroom cloud.
0:53:31 > 0:53:35It can reach 10-12 miles up into the sky.
0:53:35 > 0:53:39The cumulonimbus cloud is known as the king of clouds
0:53:39 > 0:53:42and this is because of its size
0:53:42 > 0:53:46and because of the sort of weather it produces.
0:53:46 > 0:53:51And this is the extreme end of our weather.
0:53:57 > 0:54:03The energy within one of these enormous cumulonimbus clouds
0:54:03 > 0:54:08is equivalent to the energy of ten atom bombs.
0:54:13 > 0:54:14Just amazing!
0:54:14 > 0:54:17But some clouds don't just look catastrophic
0:54:17 > 0:54:20they look almost other-worldly.
0:54:20 > 0:54:23These strange saucers have had many people looking
0:54:23 > 0:54:25for supernatural explanations.
0:54:28 > 0:54:31Lenticularis clouds.
0:54:31 > 0:54:35You would most often find them in the vicinity of mountains
0:54:35 > 0:54:41because the process that causes them to form is all to do with the wind,
0:54:41 > 0:54:46the air having to rise to pass over a mountain.
0:54:47 > 0:54:53Downwind from the peak, the air can take on this wavelike path,
0:54:53 > 0:54:55rising and dipping.
0:54:57 > 0:55:02At the peak of these waves, hovering in the wind,
0:55:02 > 0:55:04is one of these lenticular clouds.
0:55:07 > 0:55:11They can look remarkably like UFOs.
0:55:11 > 0:55:12Like disc-shaped.
0:55:12 > 0:55:16Sometimes they're stacked so you have one disc here
0:55:16 > 0:55:19and then almost a little gap and then a disc on top,
0:55:19 > 0:55:24which looks rather like the sort of pod that the aliens would sit in, I suppose.
0:55:29 > 0:55:32But there are still some clouds that even the experts
0:55:32 > 0:55:34find hard to explain.
0:55:35 > 0:55:40On the 26th of June 2012 the skies over Regina in Canada
0:55:40 > 0:55:43brought residents running out into the streets.
0:55:43 > 0:55:45We've experienced one of the coolest things
0:55:45 > 0:55:48I don't think I'll experience again in my life.
0:55:48 > 0:55:51We just experienced a wicked storm and now the sky
0:55:51 > 0:55:55has got these little puffy bubble marshmallowy things going on.
0:55:55 > 0:55:59And I probably will never experience this again in my life.
0:56:03 > 0:56:07The mama cloud, which is also known as mammatus,
0:56:07 > 0:56:14when they cover the sky, it looks almost like an Independence Day, sort of doomsday situation.
0:56:14 > 0:56:16You think, "My word, what are those about?"
0:56:16 > 0:56:21They're so kind of globular and dramatic.
0:56:21 > 0:56:27You do often see great examples out in the great plains of the States.
0:56:27 > 0:56:31You know, Tornado Alley. All the places where the stormchasers go.
0:56:34 > 0:56:39Quite how mama clouds form is not really clear.
0:56:39 > 0:56:43We're so used to seeing these different cloud forms above us,
0:56:43 > 0:56:50they're so omnipresent and yet the chaotic movements of the atmosphere
0:56:50 > 0:56:54make them really rather difficult sometimes to understand.
0:56:57 > 0:57:01As a child, I also had a fascination with those atomic bomb clouds,
0:57:01 > 0:57:04although I must confess, in a rather dark way.
0:57:04 > 0:57:08But what I didn't realise then was that clouds are heavy,
0:57:08 > 0:57:09very heavy.
0:57:09 > 0:57:12Take the average fluffy cumulus cloud.
0:57:12 > 0:57:14Using the volume, the air density
0:57:14 > 0:57:16and the concentration of water droplets,
0:57:16 > 0:57:21we can calculate it would weigh a million tonnes.
0:57:21 > 0:57:25That's the same as 200,000 African bull elephants
0:57:25 > 0:57:32or 6,200 blue whales - all just hovering right above our heads.
0:57:32 > 0:57:36These weird weather events show us that the natural world
0:57:36 > 0:57:40still has the power to surprise and keep us guessing,
0:57:40 > 0:57:44whether it's bolts of light fired down through the ash,
0:57:44 > 0:57:49or clouds that make us feel like we're under siege from above.
0:57:53 > 0:58:00What all of these stories seem to tell us is that sometimes you can't ignore the full force of nature.
0:58:00 > 0:58:03So whether we're just trying to live alongside it,
0:58:03 > 0:58:08or perhaps even contain it, it's bound to throw up a few surprises,
0:58:08 > 0:58:15curious and baffling events that are sure to have us sitting up and staring in amazement.
0:58:17 > 0:58:19Next time on Nature's Weirdest Events...
0:58:19 > 0:58:24There's the mystery of oozing sea slime...
0:58:24 > 0:58:28Bizarre body-snatchers...
0:58:28 > 0:58:30and a butterfly blizzard.
0:58:30 > 0:58:33- Do you see that, guys?- Whoa!
0:58:42 > 0:58:46Subtitles by Red Bee Media Ltd