Nearly 1,500 years ago a tsunami triggered by a rockfall swept Lake Geneva, engulfing its shores with a wall of water up to 13 metres high, Swiss scientists reported.
The incident suggests Geneva and Lausanne remain vulnerable today, as do other cities on the edge of mountain lakes and high-sided fjords, they said on Sunday.
"The risk is underestimated because most of the people just do not know that tsunami can happen in lakes," said Katrina Kremer, an earth scientist at the University of Geneva.
In a letter to the journal Nature Geoscience, Kremer's team said they delved into the "Tauredunum event", an episode that occurred in AD563.
A contemporary account by a French bishop, Gregory of Tours, described a catastrophe that was as bewildering as it was terrifying.
A giant wave charged down the lake, destroying villages and herds of animals, and then passed over the city walls of Geneva, on the western tip, where it drowned several people.
Was this a "lake tsunami"? Some experts have argued so.
They point to evidence that part of a mountain slipped into the River Rhone about five kilometres from where it flows into Lake Geneva at its eastern point.
Kremer's team swept the lake with high-resolution radar. They found a pile of sediment more than 10 kilometres long, five kilometres wide and five metres thick.
The thickest point was in the southeast, indicating an origin where the Rhone flows into the 73-kilometre-long lake.
The team then took four cores of the sediment and carbon-dated vegetal remains in it. The estimated age of the debris is between AD381 and AD612.
"Since the AD563 event is the only significant natural event recorded … within our calculated age interval, we consider our dating results to be a strong indication that the deposit is linked to the AD563 rockfall and tsunami," the letter says.
Kremer believes the rockfall caused the nearby delta's built-up slopes to collapse, with the long, narrow and crescent-shaped lake funnelling the maelstrom of mud and water into coherent, amplified waves.
"Simulations with a shallow water model show that delta collapse in the lake generates a large tsunami at various locations along the shore, where a wave of 13 metres is observed after only 15 minutes, and at Geneva where a wave of eight metres arrives 70 minutes after the mass movement," the researchers write.
A wave of this height would have had the impact on Geneva Gregory described, and today would completely inundate large parts of the inner city, they warn.
Over the past 10,000 years, several big slips have occurred, and they could happen again as sediment is building up on the delta slopes - posing a threat to more than a million people living on the lake shores, the letter says.
"Our study highlights that not only cities located on sea coasts and fjords are at risk from … tsunami, but so are densely-populated lake shores," it said.