ananyo writes "In ad 563, more than a century after the Romans gave up control of what is now Geneva, Switzerland, a deadly tsunami on Lake Geneva poured over the city walls. Originating from a rock fall where the River Rhône enters at the opposite end of the lake to Geneva, the tsunami destroyed surrounding villages, people and livestock, according to two known historical accounts. Researchers now report the first geological evidence from the lake to support these ancient accounts. The findings suggest that the region would be wise to evaluate the risk today, with more than one million inhabitants living on the lake's shores, including 200,000 people in Geneva alone. The researchers cannot say exactly what created the tsunami (nothing suggests it was an earthquake), but they propose that the falling rock caused an accumulated heap of sediment in the Rhône delta to collapse. This would have launched the wave and carried the sediment from the delta to the center of the lake, where the researchers detected it. The researchers used the geological information gathered in the study to recreate how the wave might have behaved. Their model predicted that a 13-meter-high wave would have hit Lausanne 15 minutes after the rock fall, with an 8-meter-high wave reaching Geneva after 70 minutes."