The year was 6,150 BC and in what is now the United Kingdom, a prominent human culture
lived across much of the island, primarily near the coastline.
At the time, the sea level was 15 meters or 49 feet below modern levels, and the coastline
in some areas was more than 100 kilometers towards the ocean when compared to its current
location.
The British Isles were still connected to mainland France, although this would not last
for long.
Then, the unthinkable happened.
A massive wall of water crashed into the coastline, but it did not stop there.
It was so large that it continued to advance over the low lying areas, continuing onwards
for many minutes.
Soon, this massive tsunami had reached 80 kilometers inland, wiping out potentially
hundreds to thousands of indigenous peoples who lived in small communities.
As the water finally receded, it left behind a thick white layer in the landscape which
would later be buried in a black layer of peat.
What had just occurred was the region's largest tsunami in the last 10,000 years.
It did not occur due to a large magnitude earthquake, but rather one of the planet's
most voluminous submarine landslides.
Known as the Storegga Slide, this event was caused by the collapse of 3,200 cubic kilometers
of glacial sediment and sections of Norway's continental shelf.
Although this event might seem too large to have occurred more than once, it is not an
isolated event.
Recent evidence suggests that over the last half a million years, a similarly large landslide
occurred about once every 100,000 years.
This means that a deadly repeat of this event is almost certain to occur at some point in
the future.
To understand exactly why this catastrophic landslide occurred, we must first understand
what the planet was like during the last glacial period of the ice age.
20,000 years ago, the planet was almost uniformly 7.8 degrees Celsius or 14 degrees Fahrenheit
colder than it is today, meaning that ancient London had a climate near identical to modern
day Reykjavik in Iceland.
The northern two thirds of the isles were covered in thick glaciers, and much of the
modern North Sea was also covered in ice.
Over many thousands of years, ice streams which advanced across the landscape carried
with them large volumes of rock fragments and sediment from the rocks they carved into.
Much of this material was deposited in what is called a terminal moraine at the site of
the modern North Sea Fan.
As glacial periods waxed and waned, hundreds of meters thick of sediment built up on the
steep continental edge.
Eventually, this weight reached a critical point, causing a massive landslide to begin.
As the landslide occurred, it soon propagated to other areas along the continental shelf,
propagating to a 180 mile or 290 kilometer long section of ground.
Soon, 3,200 cubic kilometers of material rapidly travelled beneath the ocean surface, where
large boulders of material would go on to travel as much as 800 kilometers distant.
This landslide displaced a significant quantity of water, leading to the generation of a towering
tsunami.
Several large waves propagated outwards from the slide, although the distribution of heights
was not exactly even.
Along the Norwegian coastline closest to the slide's origin, waves of between 12 and 14
meters inundated large swaths of the coastline.
In the Shetland Islands, the tallest waves were observed, towering to a height of more
than 25 meters.
In the Faroe Islands, waves of 14 meters were observed.
The waves would go on to strike Iceland, all of the British Isles, the Netherlands, Greenland,
and beyond with wave heights of between 2 and 19 meters.
Although the exact human toll of this ancient event is unclear, it was likely quite high.
Some sources claim that two other voluminous landslides occurred which were more than an
order of magnitude smaller in the following 100 years, but I was unable to verify this
assertion.
All I could find were further reports that a small tsunami in 3500 BC which struck the
British Isles may have also originated from a further collapse of the Storegga Slide area.
Luckily, a similarly large scale repeat of this tsunami is unlikely to occur until the
end of the next glacial period, meaning that for now the chance of a repeat event are incredibly
low.
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