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 midwinter 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 200 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 10 litres a day.
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's 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're forced upwards so they empty their moisture on the mountain slopes, leaving little for the land on the other side.
From the 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 heaviest sand rises only a few metres above the ground, but the dust can be blown 5,000 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, swallow and eddy so they pile the sand into dunes.
These sand scenes 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 300 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're being inexorably chiseled 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 k*ller.
And the red kangaroos must acknowledge that.
Right now, while the sun is low, there's 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're shielded from much of the sun's energy but their body temperature can still rise.
So they lick saliva on to 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 topsoil 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 30 degrees during the night, so it's 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's 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 the desert.
But moisture, lost even at night, has to be replaced sometime 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'd expect the place to be completely barren.
These are South America's camels, guanacos.
They're 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 the 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.
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 dry 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 tonnes 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 thought it is, has also, like the wind, shaped the land itself.
In the deserts of Utah ancient rivers flowing across sandstone country steadily widen their canyons until now the land between them has been reduced to spires and pinnacles.
With little or no soil to retain the 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 trully exhausting.
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 the 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 foof 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're 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 black fly never land, so the lizards have to leap for their food.
In one day each of these acrobatic little lizards may catch 50 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 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.
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 helpless.
Elephants may seem out of place in this landscape, but they're not the only ones.
Amazingly, lions live here, too.
In savanah country huge herds of games 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 riverbed 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.
Waterholes are filled temprorarily.
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 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 body weight every day, and a whole swarm can consume literally hundreds of tonnes 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're always heading toward 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 billions 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 is gone, the steady roar of a billion b*ating locust wings will once again be replaced by nothing more than the sound of the desert wind.