Brinicle

We’re used to seeing icicles form on tree branches and the eaves of buildings, but they can also form deep beneath the ocean, creating what’s known as a brine icicle, or brinicle.

These icy underwater tentacles are often referred to as “sea stalactites” because of their bizarre appearance, but their deadly nature has earned them another nickname: “icicles of death.”

The existence of brinicles was only discovered in the 1960s, so there’s still much to learn about them. However, scientists think life on Earth may have originated from these sea stalactites in polar seas and that they may foster conditions suitable for life on other planets and moons, such as Jupiter’s Ganymede and Callisto.

LIKE NATURAL MYSTERIES? You'll love Devil's Kettle Falls

How do they form?

When sea ice develops in the Arctic and Antarctic, impurities like salt are forced out, which is why ice created from seawater isn’t as salty as the water from which it’s formed.

As this salty water leaks from the sea ice, the surrounding water becomes more saline, lowering its freezing temperature and increasing its density. This prevents the water from freezing to the ice and causes it to sink.

As this cold brine reaches warmer seawater below, the water freezes around it, creating the descending tube of ice known as a brinicle.

When this sea stalactite reaches the seabed, a web of ice forms and spreads across it, freezing everything it touches — including any sea life it encounters, such as starfish and sea urchins — which is how brinicles earned themselves a reputation as "icicles of death."

“In areas that used to have the brinicles or underneath very active ones, small pools of brine form that we refer to as black pools of death,” Andrew Thurber, a professor at Oregon State University, told Wired. “They can be quite clear but have the skeletons of many marine animals that have haphazardly wandered into them.”

Thurber, who’s dived beneath Antarctic sea ice to collect samples, is one of the few scientists to see brinicle growth firsthand.

“They look like upside-down cacti that are blown from glass, like something from Dr. Suess’s imagination. They’re incredibly delicate and can break with only the slightest touch.”

In 2011, BBC filmmakers became the first to film brinicle formation. Using time-lapse cameras, they recorded the stunning phenomena in Antarctica in seawater that was 28 degrees Fahrenheit.

You can watch that brinicle form — and freeze everything in its path — in the video below.