Underwater icicle, a hollow tube of ice, resembling icy stalactite, progressing downward toward the seabed. Brinicles form because of the peculiarities of super cold ocean water, so the form in the extremely cold ocean environments of the Arctic and Antarctica.
When sea ice develops in the Arctic and Antarctic, impurities like salt are forced out from crystals, which is why ice created from seawater isn’t as salty as the water from which it’s formed; brinicles form around streamers of cold brine rejected by the growing sea ice. The brine itself is colder than the freezing point of seawater, since salt-rich water freezes at lower temperatures. Via osmosis — water is attracted to the brine. But the brine is so cold that the water freezes, forming a descending tube of ice.
Eventually, the tube touches bottom and the freezing touch extends outward, killing anything in its path (that’s why it’s also called “finger of death”).