Research suggests that the Sahara Desert has experienced periodic humid periods over the past 800,000 years, with some studies indicating that the region turns green roughly every 20,000 to 21,000 years. During these periods, the desert is thought to have been a more fertile and hospitable environment, with some reports suggesting that lakes and wildlife such as hippos and crocodiles once thrived in the area. The exact mechanisms and timing of these events are the subject of ongoing research and study.