Covering nearly 3.6 million square miles, the Sahara is the largest hot desert in the world and the third-largest desert overall, after the cold deserts of Antarctica and the Arctic. It spans nearly a third of the African continent, stretching from the Red Sea in the east to the Atlantic Ocean in the west. The landscape is incredibly diverse, featuring not just the iconic sand dunes (ergs), but also stone plateaus (hamadas), gravel plains (reg), and dry valleys. Temperatures can soar to over 50°C during the day and drop below freezing at night. Interestingly, the Sahara was not always a desert; thousands of years ago, it was a lush green landscape with lakes and rivers, a period known as the African Humid Period. Evidence of this past can still be seen in prehistoric rock paintings of crocodiles and hippos found in the desert's heart.