A brinicle grows from the ice sheet above

A brinicle grows from the ice sheet above. Like an "icy finger of death," this brinicle (aka brine icicle) grows from the ice sheet above in McMurdo Sound, Antarctica. Over the course of 12 hours, it descends to the seafloor below, then extends another 20 feet along the sea bed, trapping anything it touches in ice. Most creatures here move far too slowly to escape its path of death. A brinicle is formed when sea water freezes into sea ice, creating a super-salty brine. This brine percolates through cracks in the ice into the sea water below. The brine sinks because it's much denser than the surrounding water. It's also much colder, so sea water freezes on contact, forming a sinister tube of ice.

Source: Discovery.com