Imagine a desert almost as large as the U.S., and where it gets so hot you can fry an egg on the sand.
Although it may be hard to picture, it actually does exist.
Spanning the northern third of the African continent, the Sahara is the world's largest desert; it stretches from Africa's Atlantic coastline to the Red Sea.
In this vast and desolate area, the sunsets are breathtaking, the vistas are magnificent, and the sun and wind are unforgiving.
A desert is defined as an area with little or no rainfall, and in this respect, the Sahara is no exception.
It averages a mere one inch of rainfall annually, and sometimes it receives no rain for years.
There are, however, lush oases—supplied with water from underground aquifers—scattered throughout.
Temperatures can reach as high as 55 degrees Celsius during the day and drop below 0 at night.
Surprisingly, sand dunes cover only about a sixth of this desert; the rest is made up of mountain ranges, gravel plains, salt flats, and rock beds.
The Sahara wasn't always this way.
About ten thousand years ago, the Sahara received much more rainfall and was much more densely populated than it is today.
The rains created suitable growing conditions for wildlife and human settlements.
Hippos, giraffes, and even rhinoceroses roamed throughout the region.
However, about five thousand years ago, the rainfall lessened, the grasses died out, and the Sahara was transformed from a green savanna to the infertile desert that exists today.
Lack of water has created a harsh environment; nevertheless, hundreds of extremely diverse species live here.
They have evolved special adaptations to help them survive. Insects are some of the most plentiful creatures in the Sahara.
To cope with the desert's extreme temperatures, most species have a waxy covering to reduce moisture loss, and many of them burrow in the ground during daytime, coming out to eat only at night.
The Sahara scorpion is no different; it hides under rocks during the day. At night, it hunts, using the liquids from its prey to survive.
Another insect, the brightly colored scarab beetle—it can be burgundy, copper, or mint green, among other shades—has a unique adaptation.
It lays its eggs in other animals' dung, which it has collected and rolled into balls.
This behavior protects the eggs from the Sahara's abrasive winds and scorching heat; it also provides nourishment for the scarab larvae.
In ancient Egypt, people saw young beetles inexplicably emerging from the dung, and the scarab became associated with rebirth and the Egyptian god Khepri, who was responsible for the sun's daily rebirth.
Given the enormous area of the Sahara, the number of reptile species there—only about one hundred—is relatively low.
Reptiles such as snakes and lizards use many of the same methods as insects to survive the desert.
For instance, they lie in the shade offered by small rocks to keep cool. But some species have developed other, more original adaptations.
The sandfish (a type of lizard) has super-smooth scales that allow it to "swim" through sand in search of insects.
Its fringed feet also help to propel it along below the surface of the sand.
In the extreme Saharan heat, staying below the sand's surface reduces the lizard's exposure to the sun.
The Sahara also has a rather small assortment of mammals, about seventy species in all.
Over two-thirds of these are smaller animals, such as the fennec fox.
This species' thick, sand-colored fur helps protect it from extreme temperatures; even the soles of its feet are covered in fur so it can walk on the hot sand.
It also possesses big ears, which help dissipate heat and allow the fennec fox to hear insects moving on the sand.
Of the large mammals in the Sahara, antelopes, gazelles, camels, and hyenas are most common.
One of the most adaptive large animals is the addax, a member of the antelope family, which can go without drinking for several years because it gets its water from the plants it eats.
To find shelter from the heat and sandstorms, the addax digs holes in the sand dunes with its large, flat hooves.
It also uses camouflage to help it survive; babies are born the same color as the sand and stay that shade until they are strong enough to travel with the herd.
Birds use camouflage to avoid predators as well.
The Houbara bustard, a large bird with a wingspan of 140 centimeters, hunts in the rocky crags of the Sahara, where its speckled plumage helps it blend in among the rocks as it searches for rodents and lizards.
The sandgrouse's coloring also acts as camouflage, but this bird has another adaptation:
special feathers on its chest and belly that can soak up water at watering holes.
Adult birds use these feathers to bring water for their chicks from distant oases.
In the harsh environment of the desert, it is remarkable that 2.5 million people live there—mostly dwelling on the coasts or leading a nomadic lifestyle.
The most notable are the Tuareg.
Until a century ago, they controlled most of the major trans-Sahara trading routes—back when gold, ivory, and slaves were in heavy demand.
They are known for wearing the "tagelmust," a long piece of blue cloth wrapped into a turban and veil.
The tagelmust protects their faces from the sun, sandstorms, and the infamous Saharan winds.
All Tuareg men over the age of 25 must wear it—even when sleeping.
The indigo dye used to make the veil eventually wears off, rubbing into the skin and giving the Tuareg their nickname:
"the blue people."
The Tuareg have survived by adapting their needs to what little the desert can give them.
Their traditional nomadic existence involves traveling by camel caravan accompanied by their livestock, and living in tents made out of camel skins or goat hair.
They eat mostly meat and cheese made from camel or goat milk. But recent droughts have forced the Tuareg to change their lives.
Many of these people have traded their traditional tents and nomadic ways for mud-and-brick homes or farms.
One could say this new way of life is simply another adaptation to a changing world.
Part of that change includes one of the Sahara's most pressing issues:
the spread of the desert, or "desertification."
The Sahara Desert is expanding southward at a rate of about five to ten kilometers a year, and sand is encroaching on many communities and farming regions.
This is specifically a concern for the fragile savannas of sub-Saharan Africa. Desertification is intensified by human neglect.
When farmers abandon crop fields for more fertile ground, the plant roots no longer hold down the soil and the desert quickly moves in, making the land unusable.
From prehistoric times to the present, a variety of species and nomadic communities have survived by adapting to the desolation and extreme conditions of the Sahara Desert.
Surviving climate change may turn out to be their greatest challenge yet.
