The enormous prehistoric lake that once covered much of the Sahara region was Lake Mega-Chad.
Lake Mega-Chad formed when a wetter climate filled and expanded the ancient Lake Chad basin. During the African Humid Period, roughly 11,000 to 5,000 years ago, rainfall across the Sahara was far higher than today, supporting grasslands, rivers, wetlands and human settlement.
At its greatest extent, the lake covered a vast area of the central Sahara, connecting drainage systems that are now separated by desert. As the climate became drier, evaporation and reduced river flow caused the lake to shrink into the much smaller Lake Chad system known today.
Lake Mega-Chad is sometimes confused with Lake Chad itself. Lake Chad is its modern remnant, while the prehistoric lake was vastly larger and occupied parts of several present-day countries, including Chad, Niger, Nigeria and Cameroon.