Sahara Desert

The Sahara is the largest non-polar desert and the third largest desert in the world 🌎after the Antarctica and the Arctic . It's about 9,200000 square kilometres . 
The name Sahara is derived from dialectal Arabic word for desert "sahra". 
That the expands through the Algeria , Chad, Egypt , Libya , Mali, Mauritania , Morocco, Niger, Sudan, Tunisia . The boundaries of the desert are the north and the Atlantic ocean Atlas mountains to the west . The southern border is delimited by the Niger River and the Sahel. A traditional belt that transverse the continent at the line where the desert landscape transformsinto a semi arid savanah 🌵. 

Sahara has not always been a desert . After the Ice Age , the landscape was left as dry and barren as it is today . However about 10500 years ago a sudden onset of monsoon rains transformed the uninhabitable desert into a habitable savannah . As radiocarbon dated human and animal remains have proven to scientists , the humans and animals living in the Nileriver Valley were able to expand out into the Savannah . Over the next several hundred years ,the rains continued  and human settlements grew and even introduced domesticated livestock such as gots and sheeps 🐑 . Aproximantely 7,300 years ago the human population had returned to the Nile River Valley . 

The topography ofthe massive Sahara is Surprisingly varied . Roughly 25% of the desert is covered by the sand dunes . They can be small or massive often reaching over 152m in the hight . They are found in the sand seas , called ergs , where the sand is constantly shiting with the wind . Periodic rains also transform the ergs. Much of the desert landscape is comprised of rock strewn gravel plains called rega in addition to salt flats dry valleys and rocky highland plateaus called hamadas.