For the first time ever, BBC nature photographers have captured footage of an underwater brine icicle (also known as a "brinicle") slowly expanding underneath the ice of Antarctica's Little Razorback Island. Woe to be a starfish or urchin that encounters this briny deathtrap!

The trip to this filming location was precarious, as the divers had to lug heavy, current-resistant cameras while navigating narrow channels between the ice and seabed. Additionally, cavalier weddell seals would swim by and disrupt the brinicles' formation. Explains the Beeb of this phenomenon:

In winter, the air temperature above the sea ice can be below -20C, whereas the sea water is only about -1.9C. Heat flows from the warmer sea up to the very cold air, forming new ice from the bottom. The salt in this newly formed ice is concentrated and pushed into the brine channels. And because it is very cold and salty, it is denser than the water beneath.

The result is the brine sinks in a descending plume. But as this extremely cold brine leaves the sea ice, it freezes the relatively fresh seawater it comes in contact with. This forms a fragile tube of ice around the descending plume, which grows into what has been called a brinicle.

This clip was filmed as part of the Frozen Planet documentary series, whose mischievous penguins we saw previously. Hat tip to Zan!