Brinicles are long tubes of ice that form underwater, shooting downward as they freeze. They freeze so quickly, it can be observed in real time. Also known as "icicles of death," the brinicle can immediately trap and freeze any aquatic life that is in its way. Once the brinicle hits the sea floor, it creates a web of ice and kills any slow-moving sea life that may be around, such as sea urchins and sea stars.
In 2013, researchers published a study in the American Chemical Society's journal Langmuir showing just how these icy assassins form. It's a "chemical garden" process just like the one that forms hydrothermal vents, but from above: when sea ice freezes, the salt and other ions in the water doesn't freeze with it, instead accumulating in the fissures and compartments within the ice. Once the ice cracks, all that salty water, or brine, leaks out. As anyone who has sprinkled salt on a snowy sidewalk knows, salt lowers water's freezing point. There isn't a lot of water in that super cold brine, so water rushes in to fill the void. When it does, it freezes instantly, and boom—brinicle.