(Photograph) - Imagine a place where it never rains. Where the sky is clear, the sun is relentless, and there is no shade in which to shelter. A land without landmarks, where the silence is broken only by the sound of the sand shifting beneath your feet.
You are days away from civilisation, you have no water and no food. All you can see is sand, all the way to the shimmering horizon. And it's hot. Very hot. Maybe 40 degrees all day long. And when the sun sets it gets cold. Nights can be very cold in the desert.
Thank goodness for camels. The Bedouins do. Ata Allah - God's gift - they call them and the reason is this. Camels have a unique metabolic trick that allows them to survive where others would perish - they can suppress their body temperature by up to six degrees. This delays the impulse to sweat thereby reducing fluid loss. Like cacti, those other successful North American desert dwellers, they survive by storing up moisture.
And they can last a week without food or water, taking their nourishment from the hump on their back. Even when their body weight falls by a quarter they still function normally. They just keep on going, padding across the dunes on their flat, wide feet. Don't call them beasts of burden - they are bad tempered enough as it is. Camels - with their rolling left-right, left-right gait - are ships of the desert.
Desert. It's a lonely word. As a verb it means to abandon, to leave a place never to return. Mostly uninhabited, uncultivated, and infertile places, deserts are defined as anywhere on the earth's surface which sees less than 10 inches of rainfall in a year.
Vast areas of our planet - a third of the eath's surface - are deserts.
But they don't all look like this one in Mauritania. The sweeping, sandy vistas of Lawrence of Arabia's celluloid adventures account for around a quarter of desert terrain. Australia - not Africa - is the world's most arid continent and the deserts there are flat and rocky.
There's a dead calm in the desert. A famous philosophical conundrum asks: if a tree falls down in the middle of the desert and there's no one there to hear it, does it make a noise? But deserts aren't dead places. Huge dunes like these - called ergs - support small animals and insects and more than 1,000 species of plant. If and when it rains, the desert blooms.
It moves too. Saharan winds - such as the sirocco and simoom - whip up storms, sending its sands advancing by 10 kilometres a year. Extremes of temperature, combined with high winds, break down small rocks, adding to the mass of sand on its southerly journey.
The Gobi, Kalahari and Sahara are household names to us. But the Sonora, Sechura, Danakil, Tanami, Ogaden, Thar and Chalbi remain unfamiliar, faraway, forbidding places. As civilisation sends its tendrils into ever more remote parts of the world, the stubborn ecosystem of the deserts should ensure they remain our planet's purest wilderness.
Weblinks One hump or two? The A-Z of camels: www.arabnetcamelswelcome Millennium inspired joint Channel 4 Oxfam WWF education project linking places on the Greenwich meridian including Sahara desert: www.ontheline.org.uk virtual zoo of desert flora and fauna: www.livingdesert.org bucket and spade art on a big scale: www.sandsculpting.com
Picture by: Yann Arthus-Bertrand