Scientists used a combination of ice-penetrating radar flown on NASA planes and European satellite data to capture what's going on.
The cavity has an area two-thirds the size of Manhattan - and NASA researchers described the find as "disturbing". West Antarctica, home to some of the fastest-flowing and fastest-melting glaciers, accounts for the bulk of the loss calculated in the new work.
While researchers are still learning new things about the complex ways ice melts at the Thwaite Glacier, at its most basic, the giant cavity represents a simple (if unfortunate) scientific actuality.
Yet the enormous size and fast-moving growth rate of the hole in Thwaites was called both "disturbing" and "surprising" by researchers. It's big enough to contain 14 billion tons of ice, most of which has melted over the last three years.
"Thanks to a new generation of satellites, we can finally see the detail". Researchers combined the NASA data with data from Italian and German spaceborne synthetic aperture radars.
These tools revealed that the ground had shifted substantially from 1992 to 2017, the scientists found.
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"As more heat and water get under the glacier, it melts faster", study lead author and NASA scientist Pietro Milillo said. That cavern is likely filled with air much warmer than the surrounding ice, triggering faster melting of the glacier than would happen otherwise.
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. The red mass in the center shows the growing cavity.
The melting of the Thwaites Glacier, which is the approximately the size of Florida, is already behind about four percent of global sea rise, according to the Jet Propulsion Laboratory. Thwaites' grounding line is now retreating, meaning more of the glacier's underside is exposed to the water, increasing the rate of ice melt. Moreover, the glacier acts as a backstop for neighboring glaciers, meaning that it slows the rate at which they lose ice.
"Understanding the details of how the ocean melts away this glacier is essential to project its impact on sea level rise in the coming decades", added Rignot. Even with this accelerating retreat, however, melt rates on this side of the glacier are lower than on the western side.
For Thwaites, "We are discovering different mechanisms of retreat", Millilo said.
Just this week, an icebreaker ship left Chile to begin a scientific expedition to Thwaites Glacier with the help of a number of other ships, researchers, planes, and tagged wild seals. In essence, this means that the glacier at this spot is exposed to the ebb and flow of the tide, which causes the ice at the grounding line to retreat and advance across a region that's about 2 to 3 miles (3 to 5 km) long.