Most people think of tsunamis as ocean disasters — a new study shows that a lake in the Italian Alps produced two of the largest lake tsunamis ever reconstructed, and that people living nearby unknowingly helped set them up.

At Lake Iseo in northern Italy, centuries of forest clearing, grazing and mining roughly tripled the sediment flowing into the lake, the researchers report in Nature Communications. That buildup steepened an underwater delta until earthquakes in 1117 and 1222 shook it loose, sending walls of water across the lake.

A Lake That Can Make Its Own Waves

Tsunamis need a sudden push of water. At sea, that usually comes from an earthquake moving the seafloor. In lakes and fjords, it often comes from a landslide, either from a mountainside or from an underwater slope that collapses.

Lake Iseo is fed by the Oglio River, which carries snowmelt and sediment from the Val Camonica valley. Where the river enters the lake, it builds a delta whose underwater slopes descend toward the lake's deepest point, about 820 feet (250 meters) down.

Before the Middle Ages, forests and their root systems held soil in place across the watershed. After the fall of the Roman Empire, land was cleared for pasture and mining as populations grew, and the river began carrying far more sediment. Over roughly 2,000 years, that extra load made the delta's flanks steeper and less stable.

Reading the Evidence in Mud and Manuscripts

The international team, led by Muhammad Naveed Zafar of Université Savoie Mont Blanc in France, combined several kinds of evidence. Sediment cores and seismic profiles from the lake bottom revealed the scars and deposits of large underwater landslides. Archaeological records documented damage around the shore.

A 12th-century chronicle held in the Vatican Library added a human account. Describing the 1117 event, it says the lakes rose so high that very few people in nearby villages escaped, according to the authors' account on phys.org.

"Lake Iseo offers a unique natural archive," said co-author Flavio Anselmetti, a geologist at the University of Bern, in a translated statement reported by Gizmodo.

Waves Taller Than a Building

The team then simulated the collapses and the waves they would have produced. In the 1117 event, triggered by the Verona earthquake, the models suggest waves exceeded 160 feet (50 meters) along the northern lake, flooding shores in under a minute and pushing water up to about 1.7 miles (2.7 kilometers) inland.

In the 1117 model, the collapsing mass ran more than 6 miles (10 kilometers) down the lake, and its modeled deposits match what the cores and seismic profiles show. That agreement between simulation and lake-bottom evidence is what gives the team confidence in the scale of the event.

The 1222 Brescia earthquake triggered a second collapse after a century of renewed sediment buildup. That tsunami reached about 43 feet (13 meters) in the models, with water running up to about 0.6 miles (1 kilometer) inland.

Monte Isola, the large island in the middle of the lake, split the waves and partly shielded towns behind it. Even so, at the town of Iseo at the southern end, models show water rising about 18 feet (5.5 meters) in 1117, arriving roughly 6 to 7 minutes after the collapse.

The authors describe the medieval events as some of the largest landslide-induced tsunamis ever inferred in a lake. They also note an important limit: no tsunami sand was found on land, so the wave heights and inundation distances come from computer models checked against lake-bottom layers, not from direct measurements.

The Danger Has Quietly Returned

Since the 1222 collapse, more than eight centuries of sediment have built up again on the delta. Lake Iseo now has the most densely built shoreline of any lake in the Italian Alps.

"A tsunami comparable in magnitude to the catastrophic events that struck Lake Iseo during the medieval period would cause severe devastation today," the authors write, citing the region's dense population and shoreline development. They add that for towns closest to the delta, a warning system would not give enough time to react.

The lesson extends well beyond Italy. The researchers point to a global synthesis showing human-driven erosion in lake watersheds is at its highest since the Industrial Era. That suggests similar delta failures could become more likely wherever logging, farming, construction or mining sends extra sediment into steep, deep lakes in earthquake country.

Lessons for Lakes in North America

Landslide tsunamis are not only a European problem — in August 2025, a massive landslide in Alaska's Tracy Arm fjord also generated a wave that ran 1,578 feet (481 meters) up the opposite wall, the U.S. Geological Survey reported. That event was linked to glacier retreat rather than land use, but it shows how quickly steep slopes and deep water can turn dangerous.

The Lake Iseo study suggests hazard planners should consider long-term changes in sediment and land use, not just earthquake shaking. For lakeside communities, practical steps include checking whether local emergency plans account for lake tsunamis, knowing that strong shaking near a deep lake is a reason to move to higher ground, and supporting watershed management that limits erosion.

There is no sign that Lake Iseo's delta is about to fail, and the study does not forecast when another collapse might happen. The researchers' point is that the conditions for failure have been rebuilding for centuries, and that dense development has raised the stakes.

What Readers Want to Know

Can lakes have tsunamis? Yes. Landslides into a lake or collapses of underwater slopes can displace water and create tsunami waves.

What happened at Lake Iseo? Earthquakes in 1117 and 1222 triggered collapses of an underwater delta, producing two large tsunamis.

How did people contribute? Forest clearing, grazing, and mining roughly tripled the sediment entering the lake, making the delta steeper and less stable.

How big were the waves? Models suggest more than 160 feet in the northern lake in 1117 and about 43 feet in 1222. These are simulations, not direct measurements.

Could it happen again? The researchers say sediment has rebuilt over eight centuries, and a similar event today would cause severe damage.

What should lakeside residents do? Learn local emergency plans, move to higher ground after strong shaking near a deep lake, and support erosion control in the watershed.