The Sahara covers large parts of Algeria, Chad, Egypt, Libya, Mali, Mauritania, Niger, Western Sahara, Sudan and Tunisia.
It covers 9 million square kilometres (3,500,000 sq mi), amounting to 31% of Africa.
The Sahara is the world's largest low-latitude hot desert.
Hot, dry air masses primarily form over the North-African desert from the heating of the vast continental land area, and it affects the whole desert during most of the year.
Because of this extreme heating process, a thermal low is usually noticed near the surface, and is the strongest and the most developed during the summertime.
The top image shows the Safsaf Oasis on the surface of the Sahara.
The bottom (using radar) is the rock layer underneath, revealing black channels cut by the meandering of an ancient river that once fed the oasis.
The Sahara is mainly rocky hamada (stone plateaus), Ergs (sand seas - large areas covered with sand dunes) form only a minor part, but many of the sand dunes are over 180 metres (590 ft) high.