A brinicle is a hollow tube of ice that forms beneath sea ice and grows downward into the ocean, sometimes reaching the seafloor and freezing everything it touches. Often called the “icy finger of death,” it looks like an underwater stalactite made of ice, descending through the water column in real time. Brinicles form in both Arctic and Antarctic waters and can grow remarkably fast, at rates of roughly 0.2 to 2 centimeters per minute.
How a Brinicle Forms
Sea ice isn’t a solid, uniform block. As seawater freezes at the surface, it pushes salt out of the ice crystal structure, creating pockets and channels of extremely cold, extremely salty liquid called brine. This brine is denser than regular seawater and has a much lower freezing point. A saturated salt-water solution can stay liquid down to about minus 6°F, far colder than the roughly 28°F freezing point of normal seawater.
When cracks or channels in the sea ice allow this supercooled brine to drain downward into the ocean below, something dramatic happens. The brine is so cold that it freezes the less salty seawater around it on contact. A thin shell of ice forms around the sinking stream of brine, creating a fragile, translucent tube that extends further and further toward the seafloor. The brine keeps flowing through the center of this tube like water through a pipe, continuously freezing new layers of ice at the growing tip. The process is self-sustaining: as long as cold brine keeps draining from the ice above, the tube keeps extending downward.
How Fast They Grow
Brinicles grow surprisingly quickly for a natural ice formation. Field observations have recorded growth speeds ranging from about 0.2 to 2 centimeters per minute. At the faster end, that means a brinicle could extend more than a meter in under an hour. The actual speed depends on how cold the brine is, how much of it is flowing, and the temperature of the surrounding seawater. In shallow waters beneath thin, actively forming sea ice, conditions are ideal for rapid growth.
The structures themselves are delicate. The ice walls of a brinicle are thin and brittle, and ocean currents can snap them easily. Many brinicles break apart before reaching the bottom. But in calm, shallow water, they can extend all the way to the seafloor, and that’s where things get deadly for marine life.
What Happens When It Hits the Seafloor
If a brinicle reaches the bottom, the supercold brine pools and spreads across the seafloor like a slow flood. Because the brine is dense, it flows along the bottom rather than mixing upward into the water column. As it spreads, it freezes the seawater around it, creating a web of ice across the ocean floor.
Slow-moving bottom dwellers like starfish and sea urchins can’t escape in time. The spreading brine and ice encases them, freezing them in place. Any marine animal that wanders into these brine trails and pools meets the same fate. The result is a patch of frozen seafloor littered with ice-trapped organisms, a scene that looks almost surreal when captured on camera. Researchers have noted that brinicles pose a significant threat to benthic communities by rapidly encasing or destroying their habitat, a concern that becomes more relevant as climate change alters polar ice dynamics.
Where Brinicles Occur
Brinicles form in both the Arctic and Antarctic, anywhere that sea ice is actively growing and producing brine. They’re more common in areas with relatively thin, new ice, where brine channels are active and draining freely. Deep under thick, old ice, the brine drainage slows and conditions are less favorable.
Antarctic waters, particularly around the Ross Sea and beneath the ice shelves, have been a frequent site for brinicle observations. The combination of very cold air temperatures, active ice formation, and relatively shallow coastal waters creates ideal conditions. Arctic fjords and coastal zones with similar characteristics also produce them.
How They Were First Filmed
Brinicles were first described by researchers in the early 1960s and 1970s, but they remained largely unknown to the general public for decades. That changed when the BBC’s “Frozen Planet” series captured the first time-lapse footage of a brinicle forming and reaching the seafloor. The crew used underwater cameras positioned beneath Antarctic sea ice, filming over hours and compressing the footage into seconds. The resulting clip, showing a ghostly tube of ice descending to the bottom and freezing starfish in its path, went viral and earned the brinicle its dramatic nickname: the icy finger of death.
The footage made brinicles one of the most visually striking phenomena in polar science, and it remains one of the most-watched nature clips online. Researchers have since used both field observations and laboratory experiments to better understand the physics behind brinicle formation, modeling how tube diameter, brine salinity, and water temperature interact to determine growth speed and stability.
Why Brinicles Matter Beyond Spectacle
Brinicles are more than a visual curiosity. They represent an important mechanism for transporting cold, salty water from the surface to the deep ocean, contributing to ocean circulation patterns in polar regions. The brine rejection process that creates them is part of the larger thermohaline circulation system, the global “conveyor belt” of ocean currents driven by differences in temperature and salinity.
On a local scale, the ecological impact of brinicles on seafloor communities adds another variable to understanding polar ecosystems. As sea ice patterns shift with warming temperatures, the frequency, location, and intensity of brine drainage may change, potentially altering where and how often brinicles form. Whether that means more brinicles in some areas or fewer in others depends on complex interactions between air temperature, ice thickness, and ocean conditions that researchers are still working to model accurately.