A brinicle sounds like something from a sci-fi movie: a finger of ice slowly descending from above, freezing everything it touches and leaving a trail of frozen corpses. In fact it is an unusual underwater phenomenon, sometimes found in the Antarctic Ocean.
The name brinicle is a combination of brine and icicle, and they grow downwards from sheet ice. The freezing process forces out salt, creating intensely cold, salty water that does not freeze because of the salt concentration. This is heavier than normal sea water and so tends to sink.
The concentrated brine is cold enough to freeze other seawater on contact and, as it descends, forms a tube shape with the inner wall constantly melting and the outer wall freezing, growing downwards into an icy stalactite. This process is complex; they were first observed in the 1960s but not filmed until 2011 and the details are still not fully understood.
A brinicle can be 25cm in diameter and grow several metres per day. When it reaches the sea floor, it spreads out into a sheet of ice, known as anchor ice, attached to the sea bed. This ice can grow fast enough to overtake starfish, urchins and other slow-moving marine life, engulfing them and fatally freezing them to the spot.