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What's salty and cold and cool as heck? Brinicles, a rarely seen undersea phenomenon the combines ice and saltwater to become every sea star's worst nightmare! Actually, they're not that scary, just awesome. Hank explains within.

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Hank Green: Right about now, below the frozen surface of the Arctic Ocean, a column of salty liquid is flash-freezing the water around it, creating a tube of ice that can grow fast enough to be observed in real time.  As it stretches down, maybe as much as several meters, this fragile frozen dagger will make contact with the seafloor, where it'll create a fast growing web of ice that can trap and freeze any aquatic life that's too slow to get out of its way.  This otherworldly sounding formation has been called an 'icicle of death', but scientists know it as a brinicle, and it is as cool as heck, unless of course, you happen to get caught under one, in which case, it's just very cold.

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We've known about brinicles since the 1960s, but it wasn't until a few years ago that we got our first really good look at them, thanks to a BBC documentary that features some of the craziest time-laspse video you will ever see.  They're found in both north and south polar seas, usually as winter approaches and new ice begins to form.  Ocean water doesn't freeze like the water in your freezer or in a freshwater lake, when sea ice freezes on its surface, only the water freezes and the salt and other ions in the water are pushed out, leaving behind brine.  As the water continues to freeze, this salty highly concentrated brine accumulates in tiny channels and fractures within the ice, and when the ice cracks open just a bit, it gives the brine and escape route.  Because of its high salinity, this liquid brine is denser and also colder than the water below it, and since it's so salty, it freezes at a lower temperature than fresh water.  When this super cold, super saline liquid escapes from the sea ice, it begins to sink, and immediately encounters water that's very near its freezing point, but still not as cold as the brine.  And this is where all that amazing flash-freezing starts.  Our briny concoction immediately freezes any seawater it contacts as it sinks, a fragile tube of ice quickly forms around the descending plume and thickens as the brinicle stabilizes.  The process continues even if the formation hits the ocean floor, as the still delicate structure continues to freeze anything it touches.  And that's where things become a little bit dangerous for slow moving critters like sea stars and urchins.  Scientists have reported finding "black pools of death" under active brinicles or in places that used to have them, littered with the skeletons of marine animals that couldn't move fast enough as the ice enveloped them.  The study of brinicles is still in its infancy, there is still much that scientists hope to learn, but let's be patient, arctic water that's negative 2 degrees Celsius, beneath layers of sea ice, it's not exactly the most hospitable research environment.  Still, it might be worth braving those conditions just to see one of them in action, though probably I'm not gonna go check it out.

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