Frozen Planet's cameramen Hugh Miller and Doug Anderson have captured the formation of a brinicle for the first time ever. I didn't know what a brinicle was until I saw this video. I just can't believe this can happen.
Brinicles are columns of ice that form under very calm ocean conditions, when there's a big differential between the water temperature (around -1.9C) and the air temperature about the sea ice (below -20C). The warmer sea flows up to the air, freezing into new ice. According to the BBC, "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." This makes it fall down into the water, creating an ice plume that grows into the brinicle.
When it gets to the ground, it starts to expand, killing everything it touches. The whole process takes five to six hours, according to the team, which is surprisingly fast. Miller said that they didn't really know how much time it was going to take:
The one we'd seen a week before was getting longer in front of our eyes... the whole thing only took five, six hours.
You can find brinicles in the both poles, but these were filmed under the ice at Little Razorback Island, near Antarctica's Ross Archipelago. The team saw one growing in front of them while diving in that area and decided to get back to film the formation of a new one.