modern Homo sapiens to leave Africa via a route other than Sinai. The finding of saline evaporites on the floor of the Red Sea confirms that this dam has functioned at various periods in the past. Rising sea levels during the Flandrian transgression (and in earlier interglacial periods) suggest that this area may have been subject to outburst flooding. English Channel floods Originally there was an isthmus across the Strait of Dover. During an earlier glacial maximum, the exit from the North Sea was blocked to the north by an ice dam, and the water flowing out of rivers backed up into a vast lake with freshwater glacial melt on the bed of what is now the North Sea. A gently upfolding chalk ridge linking the Weald of Kent and Artois, perhaps some 30 metres (100 feet) higher than the current sea level, contained the glacial lake at the Strait of Dover. At some time, probably around 425,000 years ago and again around 225,000 years later the barrier failed or was overtopped, loosing a catastrophic flood that permanently diverted the Rhine into the English Channel and replacing the "Isthmus of Dover" watershed by a much lower watershed running from East Anglia east then southeast to the Hook of Holland and (as at modern sea level) separated Britain from the continent of Europe; a sonar study of the sea bed of the English Channel published in Nature, July 2007, revealed the discovery of unmistakable marks of a megaflood on the English Channel seabed: deeply eroded channels and braided features have left the remnants of streamlined islands among deeply gouged channels where the collapse occurred. The refilling of the Mediterranean Sea (5.3 million years ago) A catastrophic flood refilled the Mediterranean Sea 5.3 million years ago, at the beginning of the Zanclean age that ended the Messinian salinity crisis. The flood occurred when Atlantic waters found their way through the Strait of Gibraltar into the desiccated Mediterranean basin, following the Messinian salinity crisis during which it repeatedly became dry and re-flooded, dated by consensus to before the emergence of modern humans. The Mediterranean did not dry out during the most recent glacial maximum. Sea level during glacial periods within the Pleistocene is estimated to have dropped only about 110 to 120 metres (361 to 394 ft). In contrast, the depth of the Strait of Gibraltar where the Atlantic Ocean enters ranges between