In the sixth century, Gregory of Tours, a chronicler of the Germanic people known as the Franks, told of an extraordinary event in what is now Switzerland, where the Rhone River spills into Lake Geneva.

He wrote of a big rockfall in the year 563 near a place called Tauredunum. The debris plunged into the river, and a great mass of water “overwhelmed with a sudden and violent flood all that was on the banks as far as the city of Geneva,” more than 40 miles across the lake.

Historians and scientists have long believed that Gregory and another chronicler, Marius of Avenches, who told a similar tale, were describing a tsunami that raced across the lake, devastating part of Geneva and other communities along the shore. But there has never been direct evidence of it.

Researchers at the University of Geneva now say they have found that evidence, in the form of a large deposit of sediment in the middle of the lake. In a study published in the journal Nature Geoscience, they also suggest the sequence of events that caused the deadly wave: the rockfall hit the delta at the mouth of the river, causing this pile of accumulated sediment to quickly collapse into the lake and displace a huge amount of water.