One thousand years ago, on a floodplain of the Mississippi River near modern-day St. Louis, the massive Native American city known today as Cahokia sprang suddenly into existence. Three hundred years later it was virtually deserted... While analyzing cores from Horseshoe Lake, an oxbow lake that separated from the Mississippi River some 1,700 years ago, Munoz's team discovered a layer of silty clay 19 centimeters (7.5 inches) thick deposited by a massive ancient flood. It's unlikely that the ancient floodwaters were high enough to inundate the ten-story mound at Cahokia's center, a structure now called Monk's Mound... But a flood...