This clip from the BBC's Frozen Planet is one of the most amazing things you will ever see.
"Brinicle" is a clever portmanteau for an icy finger of death that forms naturally in the very cold seawater one finds around Earth's poles. A crust of sea ice can form on top of this water, and that's the first step to making a brinicle. Here's how polar oceanographer Mark Brandon explained the process in an article on the BBC website:
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.
Check out that BBC website link for more information on how the Frozen Planet videographers captured this footage. That's also where you should go to watch the video when this YouTube version is inevitably taken down.
Thank you, Brittany. Truly freaking amazing.