Brinicles are also known as an ice stalactite and they are found in both the Arctic and the Antarctic areas. That implies that in winter, the air temperature above the sea ice can below -20C whereas the sea water is about -1.9C. So, the brinicle is formed beneath the sea ice when a flow of extremely cold saline mass of water comes into an ocean warmer water. During the process, the brinicle seems an ice finger reaching down to the seafloor and the salt is concentrated into the brine channels. In fact, this newly ice formations are denser than the water beneath. Under the proper conditions originating brine pools. Often continues accumulating ice as surrounding water freezes, so the brine can advance along the seafloor freezing any bottom dwelling creatures, such as starfish and sea urchins.
Although this nature whim is known since the 1960s from model proposed by US oceanographer Seelye Martin (1974) was not filmed until 2011 by the BBC series, Frozen Planet. You can see this awesome process in a video where Sir David Attenborough describes this deathly formation using time-lapse cameras beneath the ice of foothills of the volcano Mount Erebus in Antarctic area: