Nearly 1,500 years ago a massive flood in Geneva reportedly swept away everything in its path—mills, houses, cattle, even entire churches.

Now researchers believe they’ve found the unlikely sounding culprit: a tsunami-like killer wave in the Alps. The threat, they add, may still be very much alive.

Spurred by a huge landslide, the medieval Lake Geneva “tsunami” (technically defined as a seismic ocean wave) swamped the city, which was already a trading hub, according to a new study.

Far from any ocean, the massive wave was likely generated by a massive landslide into the Rhône River, which feeds and flows through Lake Geneva, according to a group of Swiss researchers.

The team analyzed a massive sediment deposit at the bottom of the lake’s easternmost corner and determined that the material had once sat above the lake and had slid all at once into the Rhône, near where the river flows into the eastern end of Lake Geneva

The sudden splash sent a tsunami barreling down the length of the 225-square-mile (580-square-kilometer) lake toward Geneva, at the western end of the lake, the study suggests. Researchers estimate the wave was between 9 and 26 feet (3 and 8 meters) tall, depending on how quickly the rockfall occurred, which they were unable to measure.

Geneva in the Crosshairs

The Alpine tsunami, the researchers caution, isn’t just a thing of the past.

A similar event at Lake Geneva could affect the modern-day Swiss cities of Lausanne, Nyon, and Thonon-les-Bains—but Geneva itself may be at greatest risk.

The city is home to major financial and international organizations as well as nearly 200,000 people, many of whom live in low-lying areas near the lake. Unfortunately for them, the lake narrows as it approaches Geneva, creating a funnel effect that would amplify an approaching wave.

For now, there’s little indication that another Geneva tsunami is imminent, researchers have said. But the new study found evidence of several large flooding events in Geneva since the last glacier retreated from the city’s site.

“If this has happened five to six times since the last glaciation, there’s reason to believe it could happen again in the future,” said University of Geneva geologist Guy Simpson, who study team’s modeler.

“A three-meter [ten-foot] wave that hit Geneva today would be a scary wave.”