The Sahara, the world's largest desert, was once fertile grassland. This fact has been common knowledge in the scientific community for some time, but scientists are still grappling with historic data to determine whether that transition took place abruptly or gradually. At the European Geosciences Union General Assembly held in Vienna, Austria earlier this year, researchers presented new evidence showing that the eastern region of the Sahara desert, particularly the area near Lake Yoa in Chad, dried up slowly and progressively since the mid-Holocene period.