Kit Roberts
Video shows the moment a terrifying spike of ice strikes the bottom the ocean, killing anything it catches.
Many might think of Arctic and Antarctic waters as barren marine deserts, but this couldn't be further from the truth.
In fact the Arctic Ocean is teeming with life, even including coral reefs living deep beneath the ocean's surface.
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There's huge biodiversity, including the Greenland Shark, which may live as long as 400 years.
But for the less mobile creatures of the Arctic Ocean such as starfish and anemones, there is a terror stalking the arctic waters.
This isn't some horrifying marine predator, though those certainly also exist. This is a particular phenomenon which can happen in certain parts of the Arctic Ocean under sea ice.
It's called a 'brinicle', which makes it sound like some sort of awful savoury ice-lolly.
But the brinicle is even worse than that. It's a column of ice which can form, moving downwards until it hits the sea floor and freezes everything it touches to death.
The process of forming a brinicle begins within the sea ice.
The salt content in seawater sometimes creates channels of highly salty brine within sea ice as the water gets rid of the salt as it freezes.
These channels remain liquid due to the brine's lower freezing temperature than the surrounding seawater.
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Occasionally the channels of extremely cold liquid brine, cold enough to freeze water with a lower salinity, burst out into the seawater.
When this happens the brine sinks down to the bottom, as it's heavier than the water around it.
As it descends it freezes the seawater it moves through, creating a descending column of ice.
Sometimes this can hit the sea floor, where its icy tendrils spread out freezing anything too slow to get away.
While a fish or a shrimp would generally be fast enough to escape, for echinoderms like starfish or sea urchins, it could spell an icy end.
Brinicles can grow several metres a day, and spread out onto the seafloor, with the resulting ice sheet being called 'anchor ice'.
According to a study in the journal of glaciology, examples as long as six metres have been recorded in Antarctica.
This phenomenon has been known about since the 1960s, but has not been caught on film in full until 2011.
Footage of a brinicle was shown in the BBC's Blue Planet II series.
While we understand the process of how brinicles are formed, they are still not fully understood.