A third of the land on our planet is desert.
These great scars on the face of the Earth appear to be lifeless,
but surprisingly none are.
In all of them, life manages somehow to keep a precarious hold.
Not all deserts are hot.
Fifty-mile-an-hour winds,
blowing in from Siberia, bring snow to the Gobi Desert in Mongolia.
From a summer high of 50 degrees centigrade,
the temperature in mid-winter can drop to minus 40,
making this one of the harshest deserts of all.
Few animals can survive these extreme changes.
Wild Bactrian camels, one of the rarest mammals on the planet,
and perhaps the hardiest.
Their biggest problem is the lack of water, particularly now in winter
when the little there is, is locked up as ice.
Surprisingly, snow here never melts.
The air is just too cold and too dry for it to do so.
The sun's rays turn it straight into vapour.
It evaporates.
But it is the only source of water, so Bactrian camels eat it.
Elsewhere in the world, a camel at a waterhole can drink
as much as two hundred litres during a single visit.
Here the strategy is to take little and often, and with good reason,
for filling the stomach with snow could be fatal.
The camels must limit themselves to the equivalent of just ten litres a day.
LOUD MATING CALL
Winter is the time for breeding.
This extraordinary performance is a male Bactrian camel's way
of attracting the attention of a passing female.
In summer, the camels can't stray far from waterholes,
but now, with mouthfuls of snow lying everywhere,
they can travel widely in search of mates.
Today, less than a thousand of these desert specialists remain in the wild.
The Gobi, hostile though it is, is their last stronghold.
There is no other desert quite like the Gobi.
But why is this place a desert?
There is one simple and massive cause - the Himalayas.
Clouds blowing from the south hit this gigantic barrier.
As they are forced upwards, so they empty their moisture on the
mountain slopes, leaving little for the land on the other side.
From space, deserts are very conspicuous.
Dunes of sand, hundreds of miles long, streak their surface.
With no cloak of vegetation to conceal them,
strange formations are exposed in the naked rock.
Africa's Sahara is the largest desert of all.
It's the size of the United States
and the biggest source of sand and dust in the entire world.
Sandstorms like these appear without warning
and reduce visibility for days over areas the size of Britain.
Dromedaries, single-humped camels, take these storms in their stride.
The heavy sand rises only a few metres above the ground,
but the dust can be blown five thousand metres up into the sky.
The ferocious wind, armed with grains of sand, is the agent that shapes all deserts.
Reptiles have armoured scaly skins that protect them from the stinging grains.
For insects, the bombardment can be very severe indeed.
The only escape is below the surface.
As the winds rise and fall, swirl and eddy, so they pile the sand into dunes.
These sand seas can be hundreds of miles across.
In Namibia,
the winds have built some of the biggest dunes in the world.
Star dunes, like these, can be three hundred metres high.
Grains swept up the flanks are blown off the crests of the ridges, so it's only the tops that are moving.
The main body of these dunes may not have shifted for 5,000 years.
Few rocks can resist the continuous blast of the sand-carrying wind.
These outcrops are standing in Egypt's White Desert,
but they will not do so for much longer.
They are being inexorably chiselled away and turned into more sand.
Now lumps of heavily-eroded rocks have been marooned in a sea of sand.
These jagged pyramids, a hundred metres tall,
were once part of a continuous rocky plateau.
The blasting sand will eventually eliminate them altogether.
The relentless power of the wind ensures that the face of a desert
is continually changing.
But there is one constant presence...
..the desert sun.
The sun's heat and power to evaporate water has had a profound
effect on the bodies and habits of everything that lives here.
This sun, potentially, is a killer,
and the red kangaroos must acknowledge that.
Right now, while the sun is low,
there is no immediate cause for concern.
But this situation won't last long.
Australia is the world's most arid continent
with blistering daytime temperatures.
Every hour the temperature rises by five degrees centigrade.
Soon the heat will reach a critical point.
Any kangaroo out in the open is in serious danger of overheating.
In the full sun, the temperature on the ground soars to 70 degrees.
By midday, the radiation is so intense they must take shelter.
In the shade, they are shielded from much of the sun's energy,
but their body temperature can still rise.
So they lick saliva onto their forearms,
where there is a network of blood vessels
close to the surface of the skin, and as the saliva evaporates, their blood is cooled.
This thermal image shows just how effective the process is.
The blue areas on the body are the cooler parts.
As the saliva dries, it has to be replaced, and this is a real drain
on the kangaroo's body fluids.
Even in the shade, the earth is baking hot,
so the kangaroos dig away the warmed top soil to get at
the cooler ground beneath.
By staying in the shade and licking to control their body temperature,
kangaroos manage to get through
the hottest part of the day without heat stroke.
But for the majority of desert animals,
this strategy would not be enough for survival.
The extraordinary ears of the fennec foxes of Africa radiate heat,
but the animals have another way of keeping cool.
They spend their days underground and only emerge at sunset.
Darkness brings huge changes.
In the Sahara, the temperature can drop as much as
thirty degrees during the night,
so it is cool enough to allow these desert fox cubs to play.
All sorts of creatures now appear,
including some really unexpected ones.
Toads have permeable skins and would quickly die from desiccation
out in the daytime heat.
It is only now that they can leave shelter.
The same is true for scorpions,
even though their shells are actually watertight.
In fact, most small desert creatures are nocturnal, so it's only now
that you can judge just how much life there can be in a desert.
But moisture lost, even at night, has to be replaced some time,
somehow.
And that problem dominates the lives of all desert dwellers.
The Atacama in Chile. This is the driest desert in the world.
Some parts may not see rain for fifty years, and with such a record
you would expect the place to be completely barren.
These are South America's camels - guanacos.
They are very good at conserving moisture,
but they nonetheless need a regular supply of water.
They get it partly from cactus flowers,
but that explanation raises another question.
How do the cacti survive without rain?
Hot winds suck all the moisture from the surface of the land.
Clearly there must be something else that takes the place of rain.
The secret is a cold sea current that runs parallel to the land.
The cold water cools the moist, warm air above it
and that produces banks of fog.
At the same time, wind blowing on to the shore sweeps the fog inland.
Before long the cacti are dripping with dew.
The fog is so regular that moisture-loving lichens
are able to grow on the cacti and they absorb liquid like a sponge.
In a land of almost no rain,
these precious drops are life-savers for many different creatures.
Further inland, the air remains so warm that its moisture
does not condense, so this slender strip of desert
is virtually the only part of the Atacama where life can exist.
And without the fog, this land too would be empty.
The guanacos make the most of the dew,
but it will not remain for long.
In an hour or two,
the sun will have burnt it off and dried the surface of the cacti.
The Sonoran Desert in Arizona is not quite so dry as the Atacama.
Some rain does fall,
but it is infrequent and when it does arrive,
animals and plants have to be ready to make the most of it.
And it's coming.
When the summer monsoon blows in, the giant saguaros, one of the
biggest of all cacti, are ready to take full advantage of it.
After a rainstorm, the saguaro's long shallow root system sucks up the water,
and the pleats on its trunk enable it to expand rapidly.
When full, a saguaro stem can store up to five tons of water,
and that's enough to see it through many months of drought.
The trunks of these huge plants provide homes for the Gila woodpecker.
But birds are not the only animals to benefit
from the presence of the cacti.
During four weeks of the summer
the saguaros bloom - at night - to attract visitors.
The pollen and nectar with which these flowers are loaded attract
long-nosed - and long-tongued - bats.
The bats left Mexico a few days earlier to escape the heat of summer
and are on their way north to the southern United States.
To get there they have to cross the Sonoran Desert.
But the desert is so big
that for most of the year they would be unable to cross it.
Now, with the saguaro in bloom, they can refuel on the way.
So the saguaro's success in developing a way to store water
is now crucial
to most of the animals that live or even travel through this land.
The scarcity of rain determined the shape of this icon of the desert,
but water, scarce though it is,
has also, like the wind, shaped the land itself.
In the deserts of Utah,
ancient rivers flowing across sandstone country
steadily widened their canyons
until now the land between them has been reduced
to spires and pinnacles.
With little or no soil to retain water on the surface of the land,
life here is scarce indeed.
And when resources are limited, conflict is never far away.
These are Nubian ibex,
and they are squaring up for a duel.
And when trouble starts, a smart ibex knows that the best thing to do
is to gain higher ground.
These are actually subordinate male ibex
but their fights are, nonetheless, serious.
Losing one might mean never getting the chance to breed - ever.
When competitors are evenly matched, as they are here,
duels can last for an hour.
In this heat, the effort is truly exhausting,
PANTING GRUNTS
but victory here will gain important ranking points
on a male's way to the top.
There's so much at stake that not all play fair.
The battle has produced a winner,
but the ultimate prize is not his yet.
That currently belongs to the dominant male ibex.
His rank earns him the loyalty of a harem of females
and they follow him closely as he travels across this desert
searching for food and water.
He doesn't have to waste time looking for mates -
they're his for the taking.
So he can concentrate, with them, on keeping fit and healthy.
Lizards are desert specialists,
but here, their numbers are extraordinary.
These crevices in South Africa contain
the highest density of lizards in the world.
They are called flat lizards, for obvious reasons,
and they flaunt their multi-coloured bellies in territorial disputes.
He's made his point, and now it's time to find some food.
As the day warms up, the lizards move away from their cracks
and head down to the bottom of the gorge.
Their goal is the river.
There is no food at the edge, but this desert river holds a secret.
Each day, blackfly rise from turbulent stretches of the river.
This is what the lizards have come for.
The blackfly never land, so the lizards have to leap for their food.
In one day,
each of these acrobatic little lizards may catch fifty flies.
There are plenty of flies to go round,
even with hundreds of lizards competing for them.
Away from these rapids,
flat-lizard populations are found in much smaller numbers,
but here, one unusual abundance has produced another.
Deserts are created by the lack of water,
but what actually kills animals here is not heat or thirst
but lack of food.
DEEP VOCAL RUMBLE
So how on earth does a plant-eater this size survive
in a place apparently totally devoid of vegetation?
Elephants in Namibia are the toughest in Africa,
and they need to be.
What little food exists is so dispersed
that these elephants walk up to 50 miles a day as they travel
up the dry river channels searching for something to eat.
At times, the task looks truly hopeless.
Elephants may seem out of place in this landscape,
but they are not the only ones.
Amazingly, lions live here too.
In savannah country,
huge herds of game support prides containing 20 lions or more.
But to live here, lions have had to change their habits.
Prides are much smaller, and their home ranges are very much bigger.
And there's an added problem.
Their food is always on the move.
Like the elephants,
the lions must travel great distances to find enough to live on.
But lions can't go everywhere.
They won't attempt to cross this field of sand dunes,
and the oryx know it.
The lions must wait for the oryx to leave the safety of the dunes -
which eventually they must, to find food and water -
and then the lions will ambush them.
The elephants have found some of their favourite food.
Grasses are the staple diet of all elephants,
but this herd concentrates on digging up the roots,
which have more nutrition and moisture than the stems.
It's the sort of behaviour that can make all the difference
in a place of serious shortages.
Yet all this can change
in an instant.
The fortunes of many deserts are ruled by distant rains.
This water fell as rain in mountains more than a hundred miles away.
It's known as a flash flood
and called that because the water may run for just a single day.
It's an event that only happens once or twice a year at the most.
The sandy river-bed acts like a giant strip of blotting paper,
sucking up the water as soon as it appears.
But every square metre of soil moistened by this river
will increase the chances of survival
for those that live here.
Water holes are filled temporarily.
Elsewhere in Africa, elephants drink every day,
but the lack of water here means that desert elephants can only refill their tanks
once every four or five days.
Within a week the flash-flood has produced a flush of green,
more than enough to draw the oryx out of the dunes.
It's a rare chance for them to build up their food reserves.
The flood has made life easier for the lions, too.
The flesh of this oryx will keep the family going for a week at the most
but for a while, the hunting will be easier
now that the river channel has turned green.
The good times for lions and oryx are brief,
but these are the short moments that make it possible
to live in deserts the year round.
Death Valley is the hottest place on earth,
yet even this furnace can be transformed by water.
A single shower
can enable seeds that have lain dormant for 30 years or more
to burst into life.
And there hasn't been a bloom like this one for a century.
The periods of boom in Death Valley are short.
But they're just frequent enough to keep life ticking over.
A sudden flush of vegetation is what every desert dweller waits for
and, when it happens, they must make the most of it.
There is no other species on the planet that responds as quickly
and as dramatically to the good times as the desert locust.
Eggs that have remained in the ground for 20 years begin to hatch.
The young locusts are known as "hoppers"
for at this stage they're flightless.
They find new feeding grounds
by following the smell of sprouting grass.
Normally it takes four weeks for hoppers to become adults
but, when the conditions are right, as now,
their development switches to the fast track.
As the vegetation in one place begins to run out, the winged adults release pheromones -
scent messages - which tell others in the group that they must move on.
And when groups merge, they form a swarm.
An adult locust eats its entire bodyweight every day,
and a whole swarm can consume literally hundreds of tons of vegetation.
They HAVE to keep on moving.
The swarm travels with the wind.
It's the most energy-saving way of flying.
Following the flow of wind means that they are always heading towards areas of low pressure -
places where wind meets rain, and vegetation starts to grow.
As they fly, swarms join up with other swarms
to form gigantic plagues several billion strong
and as much as 40 miles wide.
They will consume every edible thing that lies in their path.
This is one of Planet Earth's greatest spectacles.
It's rarely seen on this scale and it won't last long.
Once the food has gone,
the steady roar of a billion beating locust wings will once again be replaced
by nothing more than the sound of the desert wind.
Of all the places visited by the Planet Earth team,
the most remote was Mongolia's Gobi desert.
This was the location for one of the most remarkable desert animals of all,
the elusive wild Bactrian camels.
The trip required months of planning
but arriving in Mongolia's capital, Ulaanbaatar,
was merely the start of what was to be the most challenging of shoots.
We're going for a two-month trip, and basically this area is so remote
that we take everything we need with us.
Two months' supply of food, fuel.
And in the area we are going to there is no viable source of water
so we have to bring that in, too.
It really is a quest. And the quest for camels begins today.
From Ulaanbaatar, the team were going to have to travel for a further five days
to get to where the camels live.
Despite its huge size, Mongolia has only 500 miles of paved road.
So it wasn't long before they were heading cross-country.
But even their off-road vehicles were to struggle in this terrain.
And with few vehicles, petrol stations were rather Heath-Robinson affairs,
without any of the usual safety considerations.
The Gobi desert is as large as Holland,
but surprisingly difficult to find without any roads or signs.
CONVERSATION IN MONGOLIAN AND RUSSIAN
The team were heading for the outer part of Outer Mongolia
and, in this vast, remote and rugged wilderness
they were going to have to find a group of animals
whose population numbered a mere 800.
The small community of Bayantorol was the gateway to the Gobi Desert
and home to an expert tracker called Choi Jin.
His skills were vital in the search for wild camels.
And it appeared the camels needed him, too.
He killed two wolves which...
Which killed...five?
50 camels. Two wolves killed 50 camels.
This is what he tells us now.
Choi Jin has been tracking wild camels for 50 years
so if anyone was going to get Henry to within filming distance of these elusive animals, then it was him.
Reliable vehicles were also vital to the success of the trip.
The team's Russian-made supply vehicle may have needed to be warmed up with a blowtorch each morning,
but in the event of a breakdown they would be more than covered
by the collection of spare parts brought along by the Mongolian drivers.
It was comforting to know that here was a vehicle whose engine could be rebuilt by a man with a file
in the middle of the Gobi desert.
Fortunately, they weren't depending on it for a quick getaway.
MOTOR GRINDS ENGINE KNOCKS
Since leaving Ulaanbaatar, the team had driven 1,500 miles
through the middle of Mongolia.
The supply vehicle had done the same, but they'd had to give it several days' head start.
But before leaving Bayantorol,
the team had had to deal with some local politics, as Tom explains.
Our interpreter...
decided to pick a fight
with the stand-in head of the park
and head-butted him in the face...
which is not ideal for relations.
Anyway, our driver, who was also very drunk,
decided that this was his chance to step in.
He's a big lad and...
He then punched our interpreter in the face.
Yeah, he knocked him flat.
Luckily, there were no hard feelings the following morning,
and the team were able to get on with the serious business of finding wild camels.
It wasn't long before Choi Jin spotted some promising signs.
Apparently one of the ways in which you can tell how fresh the prints are
is very, very small detail.
If he sees a small stone in the print that has got sand on it,
like that, he knows that it's very recent because otherwise that sand would have blown away.
It's very loose little grains of sand.
It's very small, a tiny size. That's why he is walking across such a large area.
Sure enough, up ahead, the team had their first sighting of wild camels.
They were already running.
Poaching has made wild camels very nervous of people.
They're about 3km or 4km away.
They spotted us from that distance. That'll be our real problem, getting close to these animals.
They are capable of spotting us from about 5km
and running for 70km in the opposite direction.
This is what is going to make this filming incredibly difficult.
But we are going to need all Henry's accomplished film skills
to get us close. We know he can do it.
What do you think? ..I don't know.
It was easy to understand Henry's doubts
when faced with the sheer scale of the landscape,
the scarcity of camels and their fear of people.
After their first sighting, the team saw nothing for five days.
So, Choi Jin told us
he had spotted...
three or four miles away, very far.
Can barely tell what it is.
Sometimes I wonder how we do it,
how he does it, because it's just watching patterns, changing patterns
and the heat haze on the horizon.
But at this distance, they weren't going to get any useful footage.
A lucky break was needed.
And an overnight snowstorm didn't seem to be it.
With the temperature plummeting to minus 20 the team - and breakfast -
needed a thawing out first.
We are having a competition to see who warms up first -
my hands or this frozen can of pilchards.
That's what it's come to.
Eating frozen pilchards straight out of the tin.
That's all we've got for breakfast.
I think I might pass.
The vehicles were useful for spotting camels but, to get close,
the team had to be on foot.
After walking hundreds of miles they were still no nearer to a sequence,
and it wasn't helped by having one less camel to film.
Gobi wolves had got to this one first.
They saw the car.
Obviously, that's a window...
The reflection, low sun.
The rear ends of running camels
continued to dominate the team's filming.
And it was beginning to cause frustrations.
So, no footage today.
They are supreme long-distance travellers,
these animals.
We are finding it difficult to keep up in the vehicle, let alone by foot.
I am absolutely knackered
and slightly annoyed that we haven't got the footage that we want the last couple of days.
I guess it's camels 1, film crew nil.
Fortunately, Choi Jin's sharp eyes remained on form.
And when combined with sheer dogged perseverance,
the team's luck finally began to change.
Not only did they start getting head-shots of camels,
but fascinating behaviour.
INTERMITTENT "CREAKY GATE" SQUEAKS
Strange mating rituals
and snow eating.
Wild camels remain one of our planet's least known animals,
so this unique footage was much needed publicity
for a species on the verge of extinction.
Subtitles by Red Bee Media Ltd
E-mail subtitling@bbc.co.uk