For the first time ever, a BBC crew has filmed the formation of a brinicle under the Antarctic sea ice, a phenomenon they're calling an "icicle of death." If you're a starfish and you see one of these coming, you'd better run— or at least, do whatever it is that starfish do when they want to get away from something.
A brinicle is kinda like an icicle, except it's formed by (not from) salt brine. Adding salt to water (like what you've got going in the ocean) lowers water's freezing point, meaning that it can stay liquid at temperatures below freezing. The more salt you add, the lower you can go. Eventually, if it gets cold enough, even salt water will freeze, but the freshest areas of water freeze first, concentrating the saltier areas together.
In the Antarctic, this kind of freezing keeps on happening until you're left with a super salty, super dense, and super cold brine. As the brine sinks downwards, it flash-freezes all of the fresher water that it comes into contact with, creating a hollow tube of ice. If the water is shallow enough, this "brinicle" extends to the sea floor, discharging the super cold brine directly onto whatever hapless starfish are lounging around down there.
Watch all the super cool action in the time-lapse video below, narrated by one of my personal heroes, David Attenborough.