“What we see before us is just one tiny part of the world. We get into the habit of thinking, This is the world, but that’s not true at all. The real world is in a much darker and deeper place than this, and most of it is occupied by jellyfish and things.”

— Haruki Murakami, The Wind-up Bird Chronicle

Haunting things happen in nature all the time. A lot of them humans have never witnessed, and for some of those that will probably always remain true.

Some, however, have become accessible to us thanks to technical advances and ingenuity. Beneath the ice off the foothills of Mount Erebus, the second highest volcano in Antarctica, a team making the documentary series Frozen Planet managed to capture on film something never witnessed by human eyes before.

An ice finger of death, reaching to the ocean floor.

Also known as a brinicle, it is the underwater equivalent of an icicle, except a little more unnerving when you see it killing things. When water freezes into ice, impurities are forced out. In seawater it means that ice doesn’t form uniformly but is instead criss-crossed by a network of extremely salty water channels.

According to Mark Brandon, polar oceanographer from The Open University, these brinicles sink to the sea floor thanks to their temperature and density:

“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.”

Watch below the incredible footage, aptly narrated by Sir David Attenborough. Unfortunately that video was taken down, but you can still see the footage here. To find out more on how they did the timelapse, head over to BBC Nature.