The cavity has two-thirds of the size of Manhattan – and NASA researchers have described the find as "disturbing".
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Researchers have previously warned that a huge Thwaites glacier could trigger the collapse of ice sheets that could increase the global sea level by 10 feet.
Scientists expected to find some gaps between ice and subsoil at Thwaites' bottom, where ocean water could flow and melt the glacier down.
Bute size and the explosive growth rate of the newly found hole surprised them.
Research scientist Eric Rignot of the University of California, Irvine, said: "For many years we have suspected that the Thwaites were not firmly connected to the subsoil beneath it.
"Thanks to the new generation of satellites, we can finally see details."
<p class = "canvas-atom canvas-text Mb (1.0em) Mb (0) – sm Mt (0.8em)MORE: Lawyers representing convicted murderer Jack Shepherd received death threats
MORE: & nbsp; RSPCA appeal after the dog was strangled to death and bound to the tree branch in the middle of the road"data-reactid =" 59 ">NEXT: Lawyers representing convicted assassin Jack Shepherd were threatened with death
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The cavity was unveiled by NASA's icebreaking icebreaker IceBridge, an air campaign launched in 2010 to explore the link between the polar regions and the global climate.
Researchers also used data from the constellations of Italian and German space-based radar satellites.
This high-resolution data can be processed by a technique called radar interferometry, which reveals how the underground area moves between the images.
"The size of the cavity under the glacier plays an important role in melting," said study director Pietro Milillo of NASA.
"When he gets more heat and water under the glacier, he's spinning faster."
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