0:00:02 > 0:00:04My name's Steve Backshall.
0:00:05 > 0:00:07Whoa!
0:00:08 > 0:00:10And I'm on a mission searching for...
0:00:10 > 0:00:12Deadly places.
0:00:12 > 0:00:13Deadly adventures.
0:00:13 > 0:00:14And deadly animals.
0:00:18 > 0:00:21And you're coming with me, every step of the way!
0:00:26 > 0:00:28Arghhhh!
0:00:32 > 0:00:33This time on Deadly,
0:00:33 > 0:00:36we head to a place where even the air is poisonous.
0:00:37 > 0:00:41Look at that! We meet a grumpy giant.
0:00:42 > 0:00:43Be careful, Steve.
0:00:45 > 0:00:49Search for a wild weirdo that's straight from hell.
0:00:49 > 0:00:53Whoa! And dangle amongst several million hot-blooded hunters.
0:00:53 > 0:00:55- Ah! - HE LAUGHS
0:00:59 > 0:01:02Last time, our journey ended in British Columbia.
0:01:02 > 0:01:06But now we've travelled 1,000 miles south to Yellowstone National Park.
0:01:09 > 0:01:11Spanning three North American states,
0:01:11 > 0:01:16it's the size of a small country and the world's first national park.
0:01:18 > 0:01:22It's home to many of the continent's most dramatic wild spectacles.
0:01:26 > 0:01:29My main mission here is to go looking for the top dog
0:01:29 > 0:01:33in North America, and my favourite animal in the world, the grey wolf.
0:01:36 > 0:01:39These are intelligent, complicated, cooperative hunters,
0:01:39 > 0:01:42with territories spanning hundreds of miles.
0:01:45 > 0:01:49Despite their ferocity, they're surprisingly shy and mostly
0:01:49 > 0:01:52nocturnal, which makes them one of the greatest filming challenges.
0:01:55 > 0:01:58Our base camp is right in the chilly heart of Yellowstone.
0:02:01 > 0:02:05To find wolves, we'll need to cover huge distances in thick snow.
0:02:11 > 0:02:13This vehicle is absolutely brilliant.
0:02:13 > 0:02:16Essentially, someone's just taken an old van,
0:02:16 > 0:02:19pulled the wheels off and welded a tank to the bottom of it.
0:02:29 > 0:02:32We've pulled up because it appears that something quite dramatic
0:02:32 > 0:02:35has happened alongside the road here.
0:02:35 > 0:02:40Loads of fur at the verge...
0:02:40 > 0:02:44and what looks like a chunk of pelt right here.
0:02:46 > 0:02:50Look at that. Wow.
0:02:50 > 0:02:56That is a fairly complete chunk of skin from an elk.
0:02:59 > 0:03:01Drag marks lead away from the road.
0:03:02 > 0:03:06That looks like it was the epicentre...of the kill.
0:03:06 > 0:03:11Yeah, oh, my goodness, there is blood everywhere, look at that!
0:03:12 > 0:03:16That's really kind of gory,
0:03:16 > 0:03:19and instantly to me this just says "wolf kill".
0:03:19 > 0:03:23You might have five or six animals all competing for food, they'll get
0:03:23 > 0:03:27their teeth stuck into it, they'll wedge down with their big paws
0:03:27 > 0:03:29on straight legs and just shake,
0:03:29 > 0:03:31using all of the muscularity in their neck,
0:03:31 > 0:03:35and it ends up kind of looking like a butcher's shop floor.
0:03:36 > 0:03:39They were here within the last couple of days.
0:03:39 > 0:03:41WOLVES HOWL
0:03:43 > 0:03:48The wolf pack is more than just a family unit, it's an organised team
0:03:48 > 0:03:51working together, to bring down prey
0:03:51 > 0:03:54far larger than any single wolf could handle.
0:03:58 > 0:04:00When prey is spotted, the lightest
0:04:00 > 0:04:02and fastest wolves take the lead...
0:04:03 > 0:04:06..while others circle around the quarry.
0:04:13 > 0:04:16Once they have their target alone and vulnerable,
0:04:16 > 0:04:18the whole pack charge in.
0:04:21 > 0:04:25It's a gruesome end for this elk, as it falls to one of the world's
0:04:25 > 0:04:29most remorseless and resourceful predators.
0:04:33 > 0:04:36Though this kill is only days old, the wolves are long gone.
0:04:39 > 0:04:41But we've had a tip off that another pack
0:04:41 > 0:04:43has been spotted 70 miles to the north.
0:04:58 > 0:05:02Our plan now is just to drive this road, keeping our eyes peeled
0:05:02 > 0:05:04for any sign of a wolf.
0:05:04 > 0:05:08Really it's all now down to my team keeping their eyes on the prize.
0:05:11 > 0:05:13We've tried to film wolves on Deadly before,
0:05:13 > 0:05:15and it's always one of our toughest challenges.
0:05:17 > 0:05:20But after a day and a half of endless driving...
0:05:21 > 0:05:23Go, go, go!
0:05:23 > 0:05:27Don't worry, just bring it round to the front with you, Stevo.
0:05:29 > 0:05:30There's two wolves!
0:05:38 > 0:05:44We've just seen a couple of wolves walking towards the road.
0:05:44 > 0:05:46At the moment, they're about a mile away,
0:05:46 > 0:05:50but it's possible they could come close enough that we can get
0:05:50 > 0:05:53a shot of them on the camera.
0:05:53 > 0:05:56The wolf's been my favourite animal for many years,
0:05:56 > 0:06:00but in the wild, I've only caught fleeting glimpses of them.
0:06:00 > 0:06:02They're so, so hard to film.
0:06:02 > 0:06:08I mean, it's not an endangered animal, there are still thousands
0:06:08 > 0:06:12of them here in North America, but no-one ever sees them.
0:06:12 > 0:06:15They are very, very shy, very elusive animals,
0:06:15 > 0:06:19and the chance of seeing one relatively close,
0:06:19 > 0:06:22in the middle of the day, is incredibly exciting.
0:06:22 > 0:06:24Let's just get off the road.
0:06:25 > 0:06:29This is great, yeah, this is great.
0:06:31 > 0:06:34Oh, wow!
0:06:34 > 0:06:35They're trotting on so quick cos...
0:06:38 > 0:06:42- (BREATHLESSLY) - Yeah, that's...fantastic,
0:06:42 > 0:06:44two wolves out in the open.
0:06:46 > 0:06:49And...really closer than I expected to get.
0:06:53 > 0:06:57So what we've got is a male and a female wolf.
0:06:57 > 0:07:00At least one of them is wearing a radio collar -
0:07:00 > 0:07:02that's put on by biologists so that they can track
0:07:02 > 0:07:05the movements of these animals and make sure they keep them safe.
0:07:05 > 0:07:08Phew. Oh, they're so beautiful.
0:07:10 > 0:07:12'I can't tell you how rare this is.
0:07:12 > 0:07:14'Some people live their whole lives in this region
0:07:14 > 0:07:18'without seeing a wolf.
0:07:18 > 0:07:22'Seeing them run free here in broad daylight is even more special,
0:07:22 > 0:07:24'because 30 years ago, they'd been wiped out
0:07:24 > 0:07:26'from most of the United States.
0:07:26 > 0:07:29'Now this top predator is making a comeback.'
0:07:31 > 0:07:33My heart's going crazy.
0:07:33 > 0:07:37Here he comes, coming almost at a trot.
0:07:37 > 0:07:39That's one of their big advantages when they're hunting,
0:07:39 > 0:07:41is their endurance. Heading towards the road,
0:07:41 > 0:07:44it looks like it might cross right in front of that car up there.
0:07:45 > 0:07:47Oh, look at that!
0:07:54 > 0:07:57The most incredible view of a wild wolf.
0:08:03 > 0:08:06Well, it was chaotic, but it was very, very exciting
0:08:06 > 0:08:08and a sight that most wildlife watchers
0:08:08 > 0:08:10would give their right arm for.
0:08:10 > 0:08:13You have to say, wolves in the winter wonderland -
0:08:13 > 0:08:15massive success and definitely Deadly.
0:08:18 > 0:08:21So the wolf may be the top of the line predator
0:08:21 > 0:08:23here in the Yellowstone snow,
0:08:23 > 0:08:27but it is nothing like the biggest or most powerful animal here.
0:08:27 > 0:08:30That creature has a skull...
0:08:33 > 0:08:34..that looks like this.
0:08:38 > 0:08:41This belongs to the largest land mammal
0:08:41 > 0:08:44found on the whole continent - the bison.
0:08:45 > 0:08:50Weighing as much as a tonne, these are true raging bulls.
0:08:50 > 0:08:52Where once they dominated a continent,
0:08:52 > 0:08:54they were ruthlessly hunted.
0:08:54 > 0:08:59Till now they only remain in protected places like this.
0:08:59 > 0:09:01In order to get close enough to film them,
0:09:01 > 0:09:02we'll need to stalk them on foot.
0:09:05 > 0:09:07Through the trees here,
0:09:07 > 0:09:09the snow's going to be very, very deep,
0:09:09 > 0:09:12possibly waist, even chest-deep, so we've got snowshoes on
0:09:12 > 0:09:16to try and keep us up above it as much as possible.
0:09:16 > 0:09:18'But even with snow shoes, we're struggling.'
0:09:20 > 0:09:21HE LAUGHS
0:09:21 > 0:09:22- Having trouble there, Aaron? - Uh?
0:09:22 > 0:09:24- Are you having trouble there? - No, it's fine.
0:09:24 > 0:09:28We're heading for a meadow where we hope the bison
0:09:28 > 0:09:31will be scraping away the snow to feed.
0:09:31 > 0:09:34Now, this is what I've been really hoping to find.
0:09:35 > 0:09:39It's a bison track, AND...look at that!
0:09:45 > 0:09:49It's... Oh, it's very fresh, it's fresher than I expected!
0:09:49 > 0:09:52That is bison poo!
0:09:53 > 0:09:54- Oh. - HE SNIFFS
0:09:54 > 0:09:57Really, really fresh, laid within the last hour or so.
0:09:57 > 0:09:59In fact, it's still warm inside -
0:09:59 > 0:10:02you could put your hands into it and use it as a glove.
0:10:04 > 0:10:06We must be close.
0:10:10 > 0:10:15'And then, through the trees, we get our first glimpse.'
0:10:17 > 0:10:20So, ahead of us is exactly what we'd hoped for.
0:10:22 > 0:10:23But once we start getting closer,
0:10:23 > 0:10:26we're going to have to be really quite careful.
0:10:36 > 0:10:38BISON GROWL AND SNORT
0:10:41 > 0:10:43Bison are bovids,
0:10:43 > 0:10:46or bovine animals, which means they're in the cow family.
0:10:46 > 0:10:50If your idea of a cow is a big friendly Friesian
0:10:50 > 0:10:53chewing the cud, then you couldn't be more wrong.
0:10:53 > 0:10:55These are creatures that are deceptively fast,
0:10:55 > 0:10:57very strong and powerful,
0:10:57 > 0:10:59and can be very grumpy and unpredictable.
0:11:01 > 0:11:05There's a huge hump which is just muscle, used to hold up that
0:11:05 > 0:11:10huge bony skull. These animals will use those in battles
0:11:10 > 0:11:14with each other, but also in fighting off their mortal enemy -
0:11:14 > 0:11:15the wolf.
0:11:18 > 0:11:20The battles between bison and wolves
0:11:20 > 0:11:24have a brutality that makes for difficult viewing.
0:11:27 > 0:11:30Both wolves and bison have strength in numbers.
0:11:30 > 0:11:34When bison stampede, the very earth shakes.
0:11:42 > 0:11:45The largest animals gather round in formation
0:11:45 > 0:11:48protecting the young and the weak.
0:11:51 > 0:11:55Often the wolves will realise they're outnumbered and outgunned,
0:11:55 > 0:11:56and will back down.
0:11:56 > 0:12:00But if they're desperate or believe they can target a weaker individual,
0:12:00 > 0:12:02they'll continue the attack.
0:12:02 > 0:12:06The wolves have guile and cooperation.
0:12:06 > 0:12:09The bison have horns that could pierce through a car door
0:12:09 > 0:12:12and kicks that can shatter bone.
0:12:12 > 0:12:14WOLF WHINES AND BISON GRUNTS
0:12:16 > 0:12:19These interactions help keep both species strong
0:12:19 > 0:12:24but are a reminder of quite how cruel life can be in this icy land.
0:12:26 > 0:12:28Oh, he's coming.
0:12:28 > 0:12:31As we return to the vehicle, we're taken by surprise
0:12:31 > 0:12:33as a mature bull steps out in front of us.
0:12:34 > 0:12:38- Everyone, come, come this way. - Be careful, Steve.
0:12:38 > 0:12:41We need to get right back to the car, guys.
0:12:45 > 0:12:48You need to back up a bit. Guys, back,
0:12:48 > 0:12:49Johnny, back.
0:12:49 > 0:12:52Maybe behind the car. Johnny.
0:12:54 > 0:12:58Bison are the most dangerous animal to people here.
0:12:58 > 0:13:01It's important not to make any sudden movements.
0:13:03 > 0:13:06And certainly, if he crashed into the side of our van,
0:13:06 > 0:13:08he'd put an almighty dent in it.
0:13:08 > 0:13:11They may not be a predator,
0:13:11 > 0:13:14but with that bulk, a top speed of 30mph
0:13:14 > 0:13:16and a reputation for grumpiness,
0:13:16 > 0:13:20they certainly qualify as a deadly defender.
0:13:24 > 0:13:27Yellowstone is a naturalist's dream.
0:13:27 > 0:13:29As you journey through the park,
0:13:29 > 0:13:32a thousand stories are left behind in the snow...
0:13:33 > 0:13:36..from the animals that have wandered there unseen.
0:13:39 > 0:13:43These trails are the tantalising proof of their presence.
0:13:44 > 0:13:46There's a really interesting trail of tracks
0:13:46 > 0:13:48running over this flat slope in front of us.
0:13:48 > 0:13:50It's going round in big circles.
0:13:50 > 0:13:54Are all very, very fresh. Certainly still in the area.
0:13:57 > 0:13:59And sure enough...
0:13:59 > 0:14:00Wow!
0:14:00 > 0:14:03..just 50 metres away, it's a red fox.
0:14:07 > 0:14:09Look at that, isn't he pretty?
0:14:09 > 0:14:12He's definitely on the hunt.
0:14:12 > 0:14:15Nose down, close to the snow.
0:14:15 > 0:14:20Listening, smelling, all of his senses on high alert.
0:14:21 > 0:14:24Red foxes are what's known as a cosmopolitan species,
0:14:24 > 0:14:26succeeding in many diverse environments.
0:14:26 > 0:14:31They are great opportunists and can find a meal almost anywhere -
0:14:31 > 0:14:33forests, deserts, even cities.
0:14:33 > 0:14:38But I've never seen them looking more majestic than here.
0:14:40 > 0:14:41That's so beautiful.
0:14:43 > 0:14:44Wow!
0:14:48 > 0:14:51The small rodents that red foxes like to snack on
0:14:51 > 0:14:53very rarely come out into the open,
0:14:53 > 0:14:56even at night at this time of year.
0:14:56 > 0:14:59Instead, they are scuttling around in tunnels underneath the snow.
0:14:59 > 0:15:01And the red fox can hear them moving.
0:15:01 > 0:15:05Even as much as my height in depth beneath the snow,
0:15:05 > 0:15:09it'll still hear their movements. The ears are that sensitive.
0:15:12 > 0:15:15Pinpointing their prey is just the start.
0:15:15 > 0:15:18Next, they have to catch it.
0:15:23 > 0:15:27Its hunting strategy is, well, as cunning as a fox.
0:15:36 > 0:15:40Punching down into the snow, it hits the jackpot.
0:15:42 > 0:15:45And an unsuspecting vole becomes lunch.
0:15:50 > 0:15:55Red foxes may have a reputation for scavenging from our backyard bins.
0:15:56 > 0:16:00Here, we see them at their most impressive, predatory best.
0:16:05 > 0:16:10Our Pole to Pole journey rolls on, heading south and east,
0:16:10 > 0:16:11to the state of North Carolina.
0:16:13 > 0:16:17It may not be cloaked in snow, but it's still plenty cold.
0:16:17 > 0:16:19We are making our way
0:16:19 > 0:16:22into the oldest mountain range on the continent...
0:16:23 > 0:16:28..in search of a surprising creature that is itself of ancient origins.
0:16:28 > 0:16:30Our next mission is taking us
0:16:30 > 0:16:33up into the Appalachian Mountains to a secret location
0:16:33 > 0:16:35where we're going in search
0:16:35 > 0:16:37of a monster that is straight out of hell.
0:16:43 > 0:16:47This rather beautiful river seems like an unlikely location
0:16:47 > 0:16:49for a slimy, nightmare monster.
0:16:49 > 0:16:52But the early settlers here clearly thought this animal was
0:16:52 > 0:16:54some kind of demon from the names they give to it.
0:16:54 > 0:16:58They called it the mud puppy, the Allegheny alligator,
0:16:58 > 0:17:01the mud devil, the snot otter.
0:17:01 > 0:17:03I think it's modern name, though, is even better.
0:17:03 > 0:17:06Nowadays it's known as the hellbender.
0:17:10 > 0:17:12The only way to find a hellbender
0:17:12 > 0:17:15is to get right into the river where they live.
0:17:15 > 0:17:18And right now, that's going to be a little bit like hell.
0:17:18 > 0:17:23This river behind us is fed not only by rain but also by a snowmelt.
0:17:23 > 0:17:25It's going to be impossibly cold,
0:17:25 > 0:17:29which is why we have a nice, thick undersuit and a dry suit.
0:17:29 > 0:17:31But I don't think even this will stop the fact
0:17:31 > 0:17:34that today is going to be pretty miserable.
0:17:39 > 0:17:40Oh, dear.
0:17:40 > 0:17:42This is going to be cold.
0:17:51 > 0:17:54Hellbenders have really suffered over the last few decades,
0:17:54 > 0:17:55with pollution, habitat loss
0:17:55 > 0:17:57and people collecting them for the pet trade.
0:17:57 > 0:17:59So they are now a protected species.
0:17:59 > 0:18:02You're not allowed to do this unless you have special permission.
0:18:02 > 0:18:06We are the working with a local hellbender survey.
0:18:06 > 0:18:10And looking under rocks like this is the perfect way to start.
0:18:14 > 0:18:17Except that this rock... is too heavy for me.
0:18:17 > 0:18:19HE LAUGHS
0:18:23 > 0:18:27Luckily, we're working with some hench local biologists
0:18:27 > 0:18:30who have the heft to give me a hand.
0:18:34 > 0:18:36I don't think we have anything under there.
0:18:38 > 0:18:41It's an intense shock, the first time you put your head under.
0:18:41 > 0:18:44There's still air in my suit, which is making me buoyant,
0:18:44 > 0:18:48and it's a real struggle to get under without a helping foot.
0:18:48 > 0:18:52- There's one here.- There's one here. Get a net.- It's coming out.
0:18:54 > 0:18:58- Straight down, straight down. Did you get it?- Lost it.
0:18:59 > 0:19:02I had a bit of a jelly fingers moment.
0:19:02 > 0:19:05Just grabbed one for a second and then...off it went.
0:19:10 > 0:19:13After four hours submerged in these freezing waters,
0:19:13 > 0:19:16I'm starting to regret suggesting this.
0:19:19 > 0:19:22I can't feel my face at all.
0:19:22 > 0:19:27And my fingers feel like someone is stabbing knitting needles into them.
0:19:31 > 0:19:35And the slippery slabs of rock are certainly not making it easy.
0:19:38 > 0:19:39OK, good, good.
0:19:45 > 0:19:47I've got one.
0:19:48 > 0:19:55I've got one. And it's a biggie. Yes!
0:19:55 > 0:19:57Fantastic!
0:19:57 > 0:19:58Now, that...
0:19:58 > 0:20:03is what all of the effort has been about - an adult hellbender.
0:20:03 > 0:20:06And it's a really good-sized tunnel.
0:20:06 > 0:20:09One of the largest species of salamander in the whole world.
0:20:11 > 0:20:15And you can kind of see where the alternative common name,
0:20:15 > 0:20:17the snot otter, comes from.
0:20:17 > 0:20:20Because...it doesn't have any scales,
0:20:20 > 0:20:22but it has a really slimy skin,
0:20:22 > 0:20:27covered with sticky mucus, which does kind of look a bit like snot.
0:20:27 > 0:20:31It's thought possible that that secretion has something in it
0:20:31 > 0:20:34which doesn't taste very good, so it's a good way of
0:20:34 > 0:20:37deterring predators, because they'll get a mouthful of that and decide
0:20:37 > 0:20:40that actually the hellbender is not a good meal.
0:20:43 > 0:20:45These animals are predators.
0:20:48 > 0:20:51Look at the size of that head, it's enormous.
0:20:51 > 0:20:56It's got this great, big, wide, smiling, gaping mouth.
0:20:56 > 0:21:00The way they catch their prey is by waiting till they get close by,
0:21:00 > 0:21:03then opening that mouth good and wide
0:21:03 > 0:21:05and sucking in, creating a vacuum
0:21:05 > 0:21:09which drags the crayfish into their mouth where it will be munched down.
0:21:11 > 0:21:15These freakishly named hellbenders can grow up to 75 centimetres long.
0:21:18 > 0:21:21Their flattened body shape means they can lurk under rocks
0:21:21 > 0:21:24ready to snatch up any passing prey.
0:21:29 > 0:21:33This wondrous creature has several other superpowers.
0:21:33 > 0:21:36One of those is the ability to regenerate parts of its body,
0:21:36 > 0:21:39to regenerate limbs, to regenerate the tail,
0:21:39 > 0:21:41so if they're snapped off by a predator,
0:21:41 > 0:21:44it can actually start to grow them back after as little as a few weeks.
0:21:44 > 0:21:48I think that is just exquisite.
0:21:48 > 0:21:52And well worth the journey, well worth the cold
0:21:52 > 0:21:53and well worth the search.
0:21:55 > 0:21:58The hellbender, the largest salamander on the continent.
0:21:58 > 0:22:01It has the ability to regenerate its limbs.
0:22:02 > 0:22:07It's covered in slimy snot, and I think it's definitely deadly.
0:22:10 > 0:22:15As I thaw out my face, we continue south, leaving the icy rivers
0:22:15 > 0:22:18and snowy mountains behind.
0:22:18 > 0:22:21So far, our Pole to Pole journey has been dominated by chill.
0:22:21 > 0:22:25But things are finally hotting up... in Texas.
0:22:30 > 0:22:32This is big country.
0:22:32 > 0:22:36In fact, the Lone Star State is the second largest in America.
0:22:36 > 0:22:40If you dropped France in the middle, it wouldn't even touch the sides.
0:22:40 > 0:22:44Big country and home to big wildlife wonders.
0:22:44 > 0:22:46Rope below.
0:22:49 > 0:22:53For at least six months of every year, Texas is home to a higher
0:22:53 > 0:22:57concentration of hot-blooded killers than any other place on the planet.
0:22:57 > 0:22:59In the dark caverns beneath our feet right now,
0:22:59 > 0:23:03there are literally millions of predators just waiting for a feed.
0:23:03 > 0:23:06It's one of the great spectacles in the natural world.
0:23:06 > 0:23:09So next step has to be into the abyss.
0:23:12 > 0:23:15By day, the cave entrance is patrolled by cave swallows
0:23:15 > 0:23:17that nest here.
0:23:17 > 0:23:19But we're heading beyond the reach of daylight
0:23:19 > 0:23:22and into the dank, dark depths.
0:23:22 > 0:23:26Into an underworld filled with a regular creepshow of killers.
0:23:28 > 0:23:30This may sound melodramatic,
0:23:30 > 0:23:33but the area in the cave is incredibly toxic.
0:23:33 > 0:23:37The diseases that dwell here have killed before.
0:23:37 > 0:23:40So, we have to cover ourselves up with these suits
0:23:40 > 0:23:43and wear gas masks. It's going to be hot, it's going to be sweaty.
0:23:44 > 0:23:46But at least we'll still be alive.
0:23:46 > 0:23:51The inside of the cave is actually hotter than the world outside.
0:23:52 > 0:23:56Warmed by the body heat of the animals we've come here to find.
0:23:58 > 0:24:01The floor is literally moving beneath our feet.
0:24:04 > 0:24:07So as not to disturb the inhabitants of the cave,
0:24:07 > 0:24:11we have to turn off our torches and switch to night vision.
0:24:12 > 0:24:14As if it wasn't creepy enough!
0:24:22 > 0:24:24Look at that!
0:24:25 > 0:24:27Absolutely phenomenal!
0:24:30 > 0:24:34It's the largest congregation of creatures ever seen on Deadly.
0:24:34 > 0:24:39That is one of the most overwhelming sights you'll ever see in nature.
0:24:40 > 0:24:45These are Mexican free-tailed bats. There are simply millions of them.
0:24:46 > 0:24:49It looks almost more like a hive of bees
0:24:49 > 0:24:51than it does a group of mammals.
0:24:56 > 0:25:00Mexicans free-tailed bats come here every year to breed.
0:25:00 > 0:25:02And Texas holds the largest colonies on earth.
0:25:05 > 0:25:08The best way to show you the intense conditions
0:25:08 > 0:25:13created by this building swarm is to use this heat-sensitive camera.
0:25:15 > 0:25:20Here in this cave, you can see that black is just the normal cave walls.
0:25:20 > 0:25:22And then you can see all of these tiny,
0:25:22 > 0:25:25little glowing embers flying through the shot.
0:25:25 > 0:25:27Those are the bats on the wing.
0:25:27 > 0:25:31But the most extraordinary thing is, if I move around,
0:25:31 > 0:25:37I get this incredible, massive, glowing flame in the centre of shot.
0:25:38 > 0:25:42And that is hundreds of thousands, millions of bats,
0:25:42 > 0:25:45crawling all over each other.
0:25:45 > 0:25:48And they have created so much body heat there
0:25:48 > 0:25:52that it's almost like one big, living radiator.
0:25:52 > 0:25:54But these bats absolutely love it.
0:25:54 > 0:25:56It creates the absolute perfect conditions
0:25:56 > 0:25:58for them to raise their young.
0:25:58 > 0:26:00Look at that!
0:26:04 > 0:26:06These bats and their young thrive down here
0:26:06 > 0:26:08in this hot, toxic environment.
0:26:09 > 0:26:12But not me. I've had enough.
0:26:12 > 0:26:14Oh!
0:26:14 > 0:26:18I have never, ever been so relieved to get out of a place.
0:26:18 > 0:26:22But if that was a vile vision of the underworld,
0:26:22 > 0:26:25what happens next is one of the greatest shows on earth.
0:26:26 > 0:26:31As night approaches, the bats rouse themselves...
0:26:31 > 0:26:34emerging to feed in a procession of millions.
0:26:36 > 0:26:40And my ropes are hanging right in the middle of it.
0:26:43 > 0:26:47Wow! The bats are just coming hurtling down the line at me.
0:26:48 > 0:26:50Ah! They're flying into my helmet!
0:26:52 > 0:26:57I have never felt quite so in the line of fire as I am right now.
0:26:57 > 0:27:00There are just squadrons of bats hurtling past me
0:27:00 > 0:27:05and clouting into me and clouting into the ropes as well.
0:27:06 > 0:27:07This is... Agh!
0:27:07 > 0:27:09HE LAUGHS
0:27:10 > 0:27:15This is the largest aggregation of mammal predators
0:27:15 > 0:27:17found anywhere on the planet.
0:27:17 > 0:27:21There could be 10 million, possibly even 12 million
0:27:21 > 0:27:24of these wonderful furry hunters heading out on the wing.
0:27:24 > 0:27:28And the amount of insects they will consume in one night
0:27:28 > 0:27:31is simply unfathomable.
0:27:32 > 0:27:36Each bat can eat up to their own bodyweight every night.
0:27:37 > 0:27:40It's thought that all the bats from this one cave
0:27:40 > 0:27:45may eat as much as 120 tonnes of insects in one night.
0:27:46 > 0:27:51Using echolocation, they'll spend the night scooping up any moth,
0:27:51 > 0:27:53midge or mosquito that gets in their path.
0:27:57 > 0:28:00So many bats. Tens of millions of them.
0:28:00 > 0:28:04All the bugs around here simply do not stand a chance.
0:28:04 > 0:28:08There is no doubt that Mexican free-tailed bats,
0:28:08 > 0:28:10emerging in their millions,
0:28:10 > 0:28:14are one of the finest predatory forces on the planet.
0:28:14 > 0:28:16And undeniably deadly.
0:28:19 > 0:28:23We've only had a tiny taste of what this huge continent has to offer.
0:28:23 > 0:28:25There's more in store.
0:28:26 > 0:28:28Next time, we'll turn up the heat.
0:28:30 > 0:28:32As we battle against tornado-force winds.
0:28:34 > 0:28:38Then cross the border into a cold-blooded Mexican fiesta.
0:28:38 > 0:28:40Oh, ya-ya-ya!