Brinicle

A brinicle (brine icicle, also known as ice stalactite) is a downward growing hollow tube of ice enclosing a plume of descending brine that is formed beneath developing sea ice.

As sea water freezes in the polar oceans, salt brine concentrates are expelled from the sea ice, creating a downward flow of dense, extremely cold and saline water with a lower freezing point than the surrounding water.  When this plume comes into contact with the neighboring ocean water, its extremely cold temperature causes ice to instantly be formed around the flow.  This creates a hollow stalactite or icicle, referred to as a brinicle.

The formation of ice from salt water produces marked changes in the composition of the nearby unfrozen water.  When water freezes, most impurities are excluded from the water crystals; even ice from seawater is relatively fresh compared with the seawater it is formed from.  As a result of forcing the impurities out, such as salt and other ions, sea ice is very porous and spongelike, quite different from the solid ice produced when fresh water freezes.

As the seawater freezes and salt is forced out of the pure ice crystal lattice, the surrounding water becomes more saline as brine leaks out. This lowers its freezing temperature and increases its density.  Lowering the freezing temperature allows this surrounding, brine-rich water to remain in a liquid state and not freeze to ice immediately.  The increase in density causes this layer to sink. Tiny tunnels called brine channels are created all through the ice as this supersaline, supercooled water sinks away from the frozen pure water. The stage is now set for the creation of a brinicle.

As this supercooled saline water reaches unfrozen seawater below the ice, it will cause the creation of additional ice. Water moves from high to low concentrations, and the brine possesses a lower concentration of water, therefore attracting surrounding water.  Due to the cold temperature of the brine water, the newly attracted water freezes.  If the brine channels are relatively evenly distributed, the ice pack grows downward evenly. However, if brine channels are concentrated in one small area, the downward flow of the cold water, now so saline that it cannot freeze at its normal freezing point, begins to interact with unfrozen seawater as a flow. Just as hot air from a fire rises as a plume, this cold, dense water descends as a plume. Its outer edges begin to accumulate a layer of ice as the surrounding water, cooled by this jet to below its freezing point, ices up. A brinicle has now been formed, resembling an inverted "chimney" of ice enclosing a downward flow of this supercooled, supersaline water.

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Wikipedia