I found this on the BBC website and found the video extremely fascinating. The only way I can describe this is a huge underwater icicle forms, which is so super cold it instantly freezes to death everything it touches.
The article which accompanies it on the BBC website is quite interesting. What you see here has never been caught on camera before.
"With timelapse cameras, specialists recorded salt water being excluded from the sea ice and sinking. 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 that froze everything it touched, including sea urchins and starfish.
The icy phenomenon is caused by cold, sinking brine, which is more dense than the rest of the sea water. It forms a brinicle as it contacts warmer water below the surface.
The unusual phenomenon was filmed for the first time by cameramen Hugh Miller and Doug Anderson for the BBC One series Frozen Planet."
It is just over a minute long. Have a look see.