'Brinicle' ice finger of death
A bizarre underwater 'icicle of death' has been captured on camera by BBC film-makers.
Using timelapse cameras, specialists recorded the salt water being excluded from the sea ice and sinking to the sea floor.
The temperature of this sinking brine, which was well below 0C, caused the water to freeze in an icy sheath around it.
Where the so-called 'brinicle' met the sea bed a web of ice formed freezing everything it touched, including sea urchins and starfish.
In this clip, Sir David Attenborough describes the first ever footage of the unusual formation.
Frozen Planet is on at 21:00 GMT on Wednesday, 23 November on BBC One