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.

Guy Simpson, an author of the study, said the thick lens-shaped layer of sediment, which lies more than 1,000 feet down in the deepest part of the lake, was found largely by chance.

Katrina Kremer, a doctoral student and the study's lead author, was conducting seismic soundings, searching for thin sediment layers that might be evidence of major floods.

“But we came across this enormous deposit,” Simpson said. “We didn't know straight away that it was the deposit that caused the tsunami. But it was a jumbled mess of sediment. It was quite obvious that it was deposited rapidly.”

The researchers then took cores of the sediments and used carbon-dating techniques on organic matter to determine when the deposit formed, narrowing the range to a period between the late fourth century and the early seventh. Other than the rockfall, there is no historical record of any special event during that period, Simpson said.

The researchers estimated that the deposit, which is at least six miles long by three miles wide and averages about 16 feet thick, contains more than 325 million cubic yards of material. They ran computer simulations that showed that the collapse of that much sediment at the mouth of the Rhone would have caused a tsunami of about 26 feet at Geneva, where it would have arrived in 70 minutes, and even higher waves elsewhere on the shoreline.

The rockfall itself may have been set off by an earthquake, as some scientists have speculated over the years.