Thwaites Glacier in West Antarctica has a problem: it has a cavity two thirds the area of Manhattan and almost 1,000 feet tall between the bottom of the glacier and the bedrock below. Unfortunately, unlike cavities in your teeth, this one can’t be filled.

The researchers expected to find some gaps between the ice of Thwaites Glacier and the bedrock because ocean water can flow in and melt the glacier from below. But the size and rapid growth of the hole surprised them: it’s big enough to have contained 14 billion tons of ice.

Most of that ice melted over the last three years.

This is just one of several disturbing discoveries reported in a NASA-led study of the glacier. NASA’s findings came from the use of ice-penetrating radar in the agency’s Operation IceBridge, an airborne campaign that began in 2010 to study connections between conditions in the polar regions and the world’s climate. The researchers also used data from Italian and German spaceborne synthetic aperture radars.

“We have suspected for years that Thwaites was not tightly attached to the bedrock beneath it,” said Eric Rignot of the University of California, Irvine, and NASA’s Jet Propulsion Laboratory. “Thanks to a new generation of satellites, we can finally see the detail.”

Thwaites Glacier is about the size of Florida and is currently responsible for about 4 percent of global sea level rise. It holds enough ice that, if it were to melt, it would raise the world ocean a little over two feet. But the glaciers around Thwaites could do even more, raising sea levels an additional 8 feet if they melt.

The cavity in Thwaites Glacier is under the main trunk of the glacier on its western side. In this region, the grounding line retreats and advances with the tides across a zone between 2 and 3 miles in size. Although the rate at which the glacier is becoming unstuck from a ridge in the bedrock has been happening at a consistent rate since 1992, the melt rate on that side of the glacier is extremely high. Interestingly, on the other side of the glacier, the rate of grounding-line retreat doubled between 1992 and 2011 and the melt rates are lower than on the western side.

What does that mean? It means that interactions between the ice and the oceans are more complex than previously understood.

“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,” Rignot said.

Photo: Thwaites Glacier. Credit: NASA/OIB/Jeremy Harbeck