With timelapse cameras, specialists recorded salt water being excluded from the sea ice and sinking.
The temperature of this sinking brine, which was well below 0C, caused the water to freeze in an icy sheath around it.
Where the so-called "brinicle" met the sea bed, a web of ice formed that froze everything it touched, including sea urchins and starfish.
The unusual phenomenon was filmed for the first time by cameramen Hugh Miller and Doug Anderson for the BBC One series Frozen Planet.
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HOW DOES A BRINICLE FORM?
Dr Mark Brandon Polar oceanographer, The Open University
Freezing sea water doesn't make ice like the stuff you grow in your freezer. Instead of a solid dense lump, it is more like a seawater-soaked sponge with a tiny network of brine channels within it.
In winter, the air temperature above the sea ice can be below -20C, whereas the sea water is only about -1.9C. Heat flows from the warmer sea up to the very cold air, forming new ice from the bottom. The salt in this newly formed ice is concentrated and pushed into the brine channels. And because it is very cold and salty, it is denser than the water beneath.
The result is the brine sinks in a descending plume. But as this extremely cold brine leaves the sea ice, it freezes the relatively fresh seawater it comes in contact with. This forms a fragile tube of ice around the descending plume, which grows into what has been called a brinicle.
Brinicles are found in both the Arctic and the Antarctic, but it has to be relatively calm for them to grow as long as the ones the Frozen Planet team observed.
The icy phenomenon is caused by cold, sinking brine, which is more dense than the rest of the sea water. It forms a brinicle as it contacts warmer water below the surface.
Mr Miller set up the rig of timelapse equipment to capture the growing brinicle under the ice at Little Razorback Island, near Antarctica's Ross Archipelago.
"When we were exploring around that island we came across an area where there had been three or four [brinicles] previously and there was one actually happening," Mr Miller told BBC Nature.
The diving specialists noted the temperature and returned to the area as soon as the same conditions were repeated.
"It was a bit of a race against time because no-one really knew how fast they formed," said Mr Miller.
"The one we'd seen a week before was getting longer in front of our eyes... the whole thing only took five, six hours."