A scar on the seafloor
Around 8,200 years ago, a section of Norway's continental shelf collapsed into the Norwegian Sea. This event, the Storegga Slide, was one of the largest submarine landslides ever identified. A section of seabed 290 kilometers long failed, displacing between 2,400 and 3,500 cubic kilometers of sediment. That is enough material to cover an area the size of Scotland to a depth of 80 meters.
The sudden displacement of this immense volume of debris generated a massive tsunami. Waves radiated across the North Atlantic, striking coastlines with devastating force. In the Shetland Islands, the nearest landmass to the slide, the tsunami run-up exceeded 20 meters—and perhaps reached as high as 25 meters—above the sea level of the time. Along the coast of mainland Scotland, the waves reached heights of 3 to 6 meters, while in western Norway they were 10 to 12 meters high. Evidence of the tsunami has been found as far away as Greenland. The cause of the slide is thought to be an earthquake, possibly combined with the rapid release of methane gas from frozen hydrates in the seafloor sediments.
The geological evidence
The tsunami left behind a clear geological marker: a distinct layer of sand, gravel, and marine sediment sandwiched between layers of local peat. This "tsunamiite" layer is the primary evidence used to reconstruct the event. In Shetland, this deposit can be seen in exposed coastal sections and has been studied in core samples from lake basins and peat bogs, such as at Sullom Voe. The deposit rests on an erosional surface, where the force of the wave scoured the existing landscape. It often contains ripped-up clasts of the peat from below, mixed with sand and marine shells.
This catastrophic event had a deep impact on the region's Mesolithic inhabitants. Coastal settlements, which were the centers of human activity, would have been completely inundated. The tsunami also likely contributed to the final submergence of Doggerland, a low-lying land bridge that connected Britain to continental Europe. While rising sea levels were already shrinking this landmass, the Storegga tsunami would have washed over its remaining islands, devastating the hunter-gatherer populations living there. The sheer scale of the flood has led some to speculate that it may be the origin of regional flood myths that were passed down through generations.