The Thwaites Glacier on Antarctica’s western coast has long been considered one of the most unstable on the continent. Now, scientists are worried about the discovery of an enormous underwater cavity that will probably speed up the glacier’s decay.

The cavity is about two-thirds the area of Manhattan and nearly 1,000 feet tall, according to a study released Wednesday by NASA’s Jet Propulsion Laboratory. The hulking chamber is large enough to have contained about 14 billion tons of ice — most of which the researchers say melted in three years.

The Thwaites Glacier, which is about the size of Florida, holds enough ice that if it all melted, it would raise the world’s oceans by over two feet, a change that would threaten many coastal cities. Climate scientists tend to watch this glacier closely, usually alongside the nearby Pine Island Glacier, which is also flowing rapidly into the Amundsen Sea.

Rising sea levels, among the most obvious threats of global warming, are caused by the melting of ice sheets, as well as the thermal expansions of the ocean. A separate study released last week found that Antarctica was contributing more to rising sea levels than previously thought.