Extensive paintings and engravings of wild animals, domesticated cattle, sheep and goats, humans and abstract symbols are found in many areas of the Sahara. These images give a fascinating insight into a lost world that is geographically and archaeologically unexpected. They are found in remote, inhospitable regions of the desert so arid that any form of sustained human or animal existence is untenable today. They document prehistoric cultures that apparently thrived in these regions, hunting wild animals and herding domesticated cattle, that have subsequently vanished leaving little trace of their presence or of the richness of their cultures. The rock-art here was created during at least the past 7,000 years against a backdrop of climatic instability and dramatic oscillations in rainfall and vegetation cover. Profound changes in the climate in North Africa over the last 12,000 years have had a massive impact on the environment and, consequently, on human occupation and subsistence. In particular, two 'wet' phases with increased rainfall and surface water enabled vegetation, animals and humans to flourish deep in the desert of today. These wet phases have been dated between about 12,000-8,000 years ago and 7,000-5 000 years ago. Periods of increased aridity, one punctuating these two wet phases and the other beginning around 5,000 years ago and becoming more pronounced towards the present, produced an expansion of desert conditions and the abandonment of immense areas of the Sahara. The earliest rock-art, much of which represents large wild animals such as elephant, giraffe, rhinoceros and a species of buffalo that is now extinct, is believed to have been created by hunter-gatherers more than 7,000 years ago and possibly as early as 10,000 BP (before present). Domesticated cattle are thought to have been widespread across the Sahara by 6,000-7,000 BP, but were possibly present earlier in certain areas, and representations of them are thought to date to this time. Subsequent styles of rock-art document the introduction of domesticated sheep and goats, horses and chariots (from around 3,000 BP), and camels (from around 2,000-2,500 BP). More recently, inscriptions in the Tuareg script, tifinagh, have been engraved at many rock-art sites. Distinct regional differences in the rock-art styles across the Sahara may indicate regional cultural differences, although other explanations for alterations in style and content in different locations should also be entertained.