We describe an enormous African Pliocene…[5.3 to 2.6 million years ago (Ma)]…paleolake…[an ancient lake]… in the valley of the River Nile. Between Juba and Khartoum,…[Sudan]… the River Nile flows through the vast Sudan Basin at the center of which is the great swamp of the Sudd…[a vast swamp in southern Sudan]…, at about 400 m elevation. Beginning in 1864, evidence accumulated that there was once an enormous lake filling the Sudan Basin. But that hypothesis foundered about 1970, apparently falsified by the absence of paleoshorelines and of any site for a natural dam, and today there is rarely any mention of it. The hypothesis is now being reconsidered after discovery of evidence from diatoms…[microalgae with silica (SiO2) cell wall…for a lacustrine…[lakes]…paleoshore line at about 515 m (El Shafie et al., 2011, J. Afr. Earth Sci.), and a spillway at the same elevation leading toward Paleolake Chad, as well as evidence from Mediterranean sapropels…[unconsolidated sedimentary deposits rich in organic material]… that a lake-draining flood may have occurred at about 3.2 Ma (Alvarez, 2023, JAES).

We now ask when the lake first came to be. Initially it looked like this question could not be answered because the evidence needed would be buried beneath the sediments of the Pliocene lake. But extensive exploration and discovery of oil has been carried out in the Sudan Basin, first by Chevron Overseas Petroleum Inc. (COPI) and more recently by other companies, especially the Chinese (CNPC), and some information on the results has been published. There have evidently been three extensional episodes — in the Early Cretaceous…[about 134 Ma]…, in the Late Cretaceous…[about 100 Ma]…, and in the Paleogene…[about 66 6o 23 Ma]…. Each episode began with deep, narrow rift valleys and ended with a broad topographic sag. Although lakes have existed at times during this evolution, there has probably not been a continuous lacustrine history. The history leading to Megalake Sudd in the strict sense appears to have begun in Neogene time…[about 23 to 2.6 Ma]…, and perhaps in the Paleogene.

The Megalake Sudd hypothesis has implications worth considering. Breaking of a natural dam and diversion of the Nile northward to the Mediterranean would have profoundly changed the drainage of the African Continent. In addition, the presence of a lake 1.5 times the area of the Caspian Sea…[located in southwestern Russia between Europe and Asia]…, today's largest lake, in the middle of North Africa should have had major climate and geographic consequences during a critical time in the evolution of Australopithecus and early Homo.