Brinicle

BBC Video of a "brinicle" in Antarctica (h/t The Dish):



The explanation:

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.

That's pretty awesome.  I thought it was cool last year when I had a bottled water supercool, then when I picked it up, it crystallized in front of my eyes.  It made me think of Ice-nine.