A brinicle (brine icicle, also known as ice stalactite) forms beneath sea ice when a flow of extremely cold, saline water is introduced to an area of ocean water, being the undersea equivalent of a hollow stalactite or icicle.
At the time of its creation, a brinicle resembles a pipe of ice reaching down from the underside of a layer of sea ice. Inside the pipe is the supercold, supersaline water being produced by the growth of the sea ice above, accumulated through brine channels. At first, a brinicle is very fragile; its walls are thin and it is largely the constant flow of colder brine that sustains its growth and hinders its melt that would be caused by the contact with the less cold surrounding water. However, as ice accumulates and becomes thicker, the brinicle becomes more stable. A brinicle can, under the proper conditions, reach down to the seafloor. On reaching the seafloor, it will continue to accumulate ice as surrounding water freezes. The brine will travel along the seafloor in a down-slope direction until it reaches the lowest possible point, where it will pool. Any bottom-dwelling sea creatures, such as starfish or sea urchins can be caught in this expanding web of ice and be trapped, ultimately freezing to death.