Deserts are areas of land with sparse vegetation, no arable farmland, and a very poor climate, usually hot and dry. Deserts often act as barriers to civilizations, as crossing a desert is often very difficult, if not impossible.  The Sahara Desert in North Africa effectively cut the southern part of Africa off from the Mediterranean world for centuries.  Also, through the process of desertification, the Sahara has been slowly claiming the arable land in Northern Africa.  Modern technology, such as the building of the Aswan Dam, has attempted to halt this process, and has met with some success.  The desert as a barrier also has positive effects.  The Sahara protected Southern Africa from invaders from the north, while also providing a secure western flank to the Egyptians.  It is not until World War II that serious warfare can occur across the desert.

The Gobi Desert in China has acted as a barrier to both invasion and cultural diffusion.  China developed a very ethnocentric outlook due to their isolation.  Contributing to this isolation are rainforest and mountains.  The Gobi Desert has also forced most of China's people to live in the more fertile east, as no good farmland exists.  Despite the negatives, the Gobi Desert is a fantastic resource of fossils.  Many different species of dinosaurs have been identified from remains found in this desert, including an Oviraptor, sitting on a nest of fossilized eggs.