The Sahara Desert is the world’s largest hot desert and one of the harshest environments on the planet. It is third-largest desert overall after Antarctica and the Arctic, which are cold deserts.

Blanketing much of the northern third of the African Continent, or some 3.5 million square miles, the Sahara Desert extends eastward from the Atlantic Ocean some 3,000 miles to the Nile River and the Red Sea, and southward from the Atlas Mountains of Morocco and the Mediterranean shores more than 1,000 miles to the Savannah called the Sahel. More than 16 times the size of France, the Sahara Desert blankets nearly all of Mauritania, Western Sahara, Algeria, Libya, Egypt and Niger; the southern half of Tunisia; and the northern parts of Mali, Chad, and Sudan.

The Sahara has only two permanent rivers and a handful of lakes, but it has substantial underground reservoirs or aquifers. Its permanent rivers are the Nile and the Niger. The Nile rises in central Africa, south of the Sahara, and flows northward through Sudan and Egypt and empties into the Mediterranean. The Niger rises in Western Africa, southwest of the Sahara, and flows northeastward into Mali and the desert then turns southeastward, through Nigeria, and empties into the Gulf of Guinea.

The Sahara has some 20 or more lakes, but only one with potable water–the expansive but shallow Lake Chad, a continually expanding and shrinking body of water that lies in the country of Chad, at the southernmost edge of the Sahara. Other lakes hold a briny stew of undrinkable water.

Although hot and extremely dry today, it is believed that the Sahara Desert has undergone various climatic shifts for the last few hundred thousand years. For example, during the last glaciation, it was bigger than it is today because precipitation in the area was low. But from 8000 BCE to 6000 BCE, precipitation in the desert increased because of the development of low pressure over ice sheets to its north.

Once these ice sheets melted, however, the low pressure shifted and the northern Sahara dried out but the South continued to receive moisture due to the presence of a monsoon. Theories that the Sahara was once home to waterways that sustained life from rhinos to humans and various species of fish have gained traction in the last few years. In 2013, researchers argued that based on computer modelling of the Sahara as it existed 100,000 years ago, monsoon rains would have been heavy enough to feed three main rivers.

Some paleohydrologists believe these waterways are the key to the answer of how humans migrated out of central Africa.

People sometimes can’t get their head around climate change and how quickly it happens. Here’s an example where within just a couple of thousand years, the Sahara went from being wet and humid, with lots of sediment being transported into the canyon, to something that’s arid and dry – Russell Wynn, a scientist with the National Oceanography Center.

As little as 6,000 years ago, the vast Sahara Desert was covered in grassland that received plenty of rainfall, but shifts in the world’s weather patterns abruptly transformed the vegetated region into some of the driest lands on Earth. A Texas A&M University researcher is trying to uncover the clues responsible for this enormous climate transformation – and the findings could lead to better rainfall predictions worldwide.

The Sahara could be home to life and vegetation again. The researchers estimate that the river system held water nine times over the past 200,000 years with periods of humidity and climate change occurring every 20,000 years.