Archeologists have long sought to prove that the great flood described in Genesis and in the Babylonian epic of Gilgamesh was a historic event. Columbia University geologists Ryan and Pitman weigh in with a highly conjectural theory that seems as good as any other, if no better. Around 5600 B.C., they maintain, Noah's flood occurred when rising Mediterranean waters roared through the narrow Bosporus Strait, transforming the Black Sea, then a freshwater lake, into a bloated saltwater body. Taking a cue from Australian prehistorian Gordon Childe, who posited that Europe's first farmers came from Asia, the authors contend that the Black Sea at the time of the alleged flood was a fertile oasis, a cultural magnet where diverse peoples?farmers, animal breeders, artisans?exchanged techniques and possibly genes. They point to the sudden appearance in Europe, shortly after 5600 B.C., of