The bottom of Lake Como is 200 meters below sea level, and the reason is that the Mediterranean Sea once evaporated almost completely.Around 5.5 million years ago, tectonics sealed the Strait of Gibraltar. Cut off from the Atlantic, the Mediterranean dried into a salt desert more than a kilometer below today's coastlines. Geologists call it the Messinian Salinity Crisis.With the sea gone, every river flowing south off the Alps suddenly had a kilometer of extra elevation to fall. They responded t