As sea ice freezes in the Antarctic, it leaves behind a concentrated brine which is trapped in channels within the ice. When this cold, dense brine, which is colder than the freezing point of water, seeps to the underside of the ice layer and sinks into the water beneath, it forms a sheath of frozen water around it, which eventually extends to the sea floor as a 'brinicle'.
For the BBC Nature series Frozen Planet, these brinicles were filmed for the very first time. The film's producer, Kathryn Jeffs, said that when she was preparing to film them, she couldn't find any information about them, even on the Internet.
Now, this video is among the most popular nature video clips online.
Jeff describes the technically demanding and physically difficult process of filming under thick ice in 2ºC water. As she put it, they were tremendously satisfied when they
... knew [they] had captured, for the first time ever, the creation of a rather sinister wonder of nature.