Filmed "beneath the ice off the foothills of the volcano Mount Erebus", the timelapse covers a period of 4-5 hours; here's some of the relevant science (from the YouTube link):
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.I was surprised to see that it killed starfish on the sea floor. The video is excerpted from the "Frozen Planet" series. More information at the BBC.
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.