Divers wait expectantly as a finger of ice extends downwards from the ice flow above. It creeps quickly, its growth at times visible to the naked eye. When it reaches the sea floor, ice crystals expand from the underwater icicle to engulf unsuspecting starfish and sea urchins feeding nearby. What is this strange phenomenon? It is a rare, recently discovered ice structure known as a brinicle.

What is a Brinicle?

The salty water of the ocean has a lower freezing point than fresh water. While fresh water freezes at zero degrees Celsius, while seawater in the arctic and antarctic regions has been recorded at negative 1.9 degrees, and colder. When the frigid air temperatures do chill the water enough to freeze, the crystallization that takes place also differs from that of fresh water. Instead of a solid mass of ice, sea water often forms a spongy crystalline structure with channels of concentrated brine, or salt water, within it.

Since this brine is denser and heavier than the surrounding sea water, it pools and migrates through cracks in the the floating sea ice. When it reaches the less concentrated water below, it begins to sink. As the brine is also colder than the water through which it is passing, the sea water freezes on contact and crystals of ice form around it. When these ice crystals form a tube around a descending column of brine, a brinicle is formed. The word brinicle is a contraction of “brine icicle.” They are also called “sea stalactites” due to their resemblance to this common cave structure. Brinicles, however, are not hard like rock. The crystals are delicate and can be broken by the slightest touch.

When the brinicle reaches the sea floor, the cold brine expands outward across the seabed. A web of ice crystals may form along this progression, resembling a river of ice. The entire process generally takes between four and twelve hours.

Are Brinicles Dangerous?

Brinicles are not dangerous to humans, as man seldom travels beneath the ice sheets where they form. Divers who study brinicles take precautions to avoid hypothermia or other cold water injuries.

However, slow moving, bottom dwelling creatures such as starfish and sea urchins are unable to move quickly enough to escape the flow of brine. They may become encased in ice, completely frozen by the brine. That is how brinicles earned the title “icicles of death.” Pools of super-cold brine may also form and remain beneath the site of brinicle formation. These so-called “black pools of death” can also be deadly to small sea creatures that wander through them.

Where do Brinicles Form?

Brinicles form beneath floating sea ice. Because of the intensely cold temperatures involved, their appearance on earth is limited to the polar regions, both the Arctic and Antarctica.

According to a study published in the American Chemical Society’s journal, Langmuir, the formation of brinicles mirrors that of the undersea hydro-thermal vents – the primary difference being water temperature. The brinicles are similar to the vents in that they, too, may be unlikely hotbeds of life, due to the rich mineral and lipid content of the brine. Some scientists like Catherine Coath speculate that life, such as microscopic extremophiles, may be found near similar formations on other planets, such as on Jupiter’s icy moons.

Recording of a Brinicle Formation

In 2011, the crew of the Discovery Channel series Frozen Planet became the first to film a brinicle formation. They used time lapse photography to capture the event over the course of several hours. Their first attempt was unsuccessful, as large seals in the area knocked over camera equipment and broke fledgling brinicle structures.