Scientists expected to find some relatively small gaps between the glacier and bedrock, but were unsettled by the 1,000-foot deep cavity the mission revealed. It is growing at an "explosive" rate that surprised researchers conducting a study the agency led on the glacier.
The study was published Wednesday in the peer-reviewed journal Science Advances.
Scientists spotted the concealed void thanks to a new generation of satellites, Rignot noted. They've also used data from a constellation of Italian and German radars.
"[The size of] a cavity under a glacier plays an important role in melting", said Pietro Milillo of the Jet Propulsion Laboratory.
Thwaites Glacier alone holds enough ice above sea level to raise sea levels by more than 65cm if it was to melt.
Worse, Thwaites Glacier acts as a kind of "door stop", preventing adjoining glaciers from sliding towards the sea.
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It warned that a failure to resolve global trade tensions could further destabilise a slowing global economy. But that's a far cry from the 0.4% growth in the first two quarters of the year.
The cavity is 300m tall and is growing fast. Understanding this new quirk in the glacier's underbelly will be crucial to refining estimates of how fast the overall glacier will retreat. The glacier itself is about as big as the state of Florida, and, if melted completely, could raise sea levels by some 2 feet globally, the scientists say.
That's important to know, since Thwaites now accounts for about 4 percent of global sea level rise. The International Thwaites Glacier Collaboration will begin its field experiments in the Southern Hemisphere summer of 2019-20.
Recently, there have been repeated reports about the melting of the ice in the Antarctic.
The new data, revealed by ice-penetrating radar aboard aircraft flying over Antarctica, point to a previously underestimated method of glacial collapse that scientists' models don't adequately factor in.
Another changing feature is a glacier's grounding line - the place near the edge of the continent where it lifts off its bed and starts to float on seawater. This description has resulted in the glacier being dubbed the "most unsafe glacier in the world." . That exposes more of a glacier's underside to sea water, increasing the likelihood its melt rate will accelerate.
In the case of the recently discovered cavity, it was found on the western side under the main trunk of the glacier.
Since 1992, it has been slowly becoming unstuck from the bedrock at a rate of 0.4 miles to 0.5 miles a year, and the melt rate is considered extremely high. But the research shows melt in East Antarctica-long thought to be the more stable region-has been underestimated. As they are melting and the formation of icebergs, the water will be even greater. The fastest retreat of floating ice is about a half mile a year with various areas thinning at up to 650 feet per year.