Storegga Slide

The three Storegga Slides are amongst the largest known submarine landslides. They occurred under water, at the edge of Norway's continental shelf in the Norwegian Sea, approximately 6225–6170 BCE. The collapse involved an estimated length of coastal shelf, with a total volume of of debris, which caused a megatsunami in the North Atlantic Ocean.
Description.
Storegga (Norwegian: "Great Edge") is located at the edge of Norway's continental shelf in the Norwegian Sea, north-west of the Møre coast. In around 6200 BCE, structural failures of the shelf caused three underwater landslides, which triggered very large tsunamis in the North Atlantic Ocean. The collapses involved an estimated length of coastal shelf, with a total volume of of debris. 
Based on carbon dating of plant material recovered from sediment deposited by the tsunamis, the latest incident occurred around approximately 6225–6170 BCE. In Scotland, traces of the subsequent tsunami have been recorded, with deposited sediment being discovered in Montrose Basin, the Firth of Forth, Scotland, up to inland and above current normal tide levels.
Possible mechanism.
A possible triggering mechanism is thought to have been an earthquake physically triggering a catastrophic expansion of methane clathrate. A cubic metre of solid clathrate expands to 164 cubic metres of methane.
Another theory is that streams from melting glaciers had carried trillions of tons of sediment to the edge of the continental shelf, and that a trigger such as an earthquake caused a large area of seafloor to collapse into the deep Norwegian sea.
Impact on human populations.
At, or shortly before, the time of the Second Storegga Slide, a land bridge known to archaeologists and geologists as "Doggerland" existed, linking Britain, Denmark and the Netherlands across what is now the southern North Sea. This area is believed to have included a coastline of lagoons, marshes, mudflats, and beaches, and to have been a rich hunting, fowling, and fishing ground populated by Mesolithic human cultures.
Although Doggerland was permanently submerged through a gradual rise in sea level, it has been hypothesized that coastal areas of both Britain and mainland Europe, extending over areas which are now submerged, would have been temporarily inundated by a tsunami triggered by the Storegga Slide. This event would have had a catastrophic impact on the contemporary Mesolithic population. 
Although the tsunami caused by the Second Storegga Slide would have been devastating for those within the run-in zone, ultimately the tsunami was neither universally catastrophic nor the reason behind the inundation of the last vestiges of Doggerland.
Modern day impact.
As part of the activities to prepare the Ormen Lange natural gas field, the incident has been thoroughly investigated. One conclusion is that the slide was caused by material built up during the previous glacial period and that a recurrence would be possible only after a new ice age. Facts and arguments supporting this conclusion were made public in 2004, and thus it was concluded that the development of the Ormen Lange gas field would not significantly increase the risk of triggering a new slide.