A hollow tube of ice called a brinicle freezes everything it touches below the sea
As polar sea ice forms, it expels dense, supercooled brine that sinks through the ocean below. Where that plume meets seawater, ice freezes around it as it falls, building a hollow stalactite called a brinicle that keeps growing toward the seafloor. Any starfish or sea urchin it reaches gets encased in spreading ice and freezes to death. The process was first filmed in 2011, by the BBC's Frozen Planet team in Antarctica.