In very cold sea water, dense brine sinks to the bottom. It is much colder than the water around it and a sleeve of ice forms around the rivulet of brine. This is a brinicle and, when it touches the sea bottom, it freezes and kills everything it touches.
This amazing video is of an Antarctic brinicle. It took between five and six hours for the brinicle to extend from the surface ice to the sea floor. Here, with time-lapse photography, it takes less than two minutes. (via MetaFilter)