WATCH: A brinicle, or 'icicle of death,' grows in the ocean

When is an icicle not an icicle?

How about when it's growing underneath an ice shelf in the Antarctic Ocean and is as salty as a bag of movie popcorn?

Witness the weird life of a "brinicle," a spiraling cone of ice built around freezing particles of salt water. A film crew from the BBC collaboration Frozen Planet has captured the first-ever footage of a brinicle growing, which you can see below in wonderful time-lapse. These frigid formations are thought to sprout regularly in the subzero waters of the poles, but are difficult to observe because of hazardous diving conditions and the interference of curious seals. (Seriously.) To film this brinicle, Frozen Planet's Hugh Miller and Doug Anderson had to use a Rube Goldbergian rig that looked like this:

Brinicles can sprout when calm salt water becomes colder than the surrounding sea and begins to drift toward the ocean floor. The frosty brine instantly freezes the water around it, creating a tornado-looking structure that can spread across the sand below to engulf nearby marine animals, killing them or at least locking them in an annoying state of suspended animation. In this case, the brinicle, located under Little Razorback Island, takes out a horde of sea urchins and starfish. The whole process took five to six hours.

For a brinicle discussion by Open University oceanographer Mark Brandon, follow the jump.

According to Brandon:

Freezing sea water doesn't make ice like the stuff you grow in your freezer. Instead of a solid dense lump, it is more like a seawater-soaked sponge with a tiny network of brine channels within it.

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.

Brinicles are found in both the Arctic and the Antarctic, but it has to be relatively calm for them to grow as long as the ones the Frozen Planet team observed.