A massive glacier at the edge of the West Antarctic Ice Sheet is cracking from the inside out at accelerating speed. That’s alarming because this glacier — and others — function like corks in a bottle: they keep the ice from flowing into the sea, which would raise sea levels by several feet.

The glacier, which is described in a paper published yesterday in Geophysical Research Letters, is called Pine Island Glacier. In 2015, a 224-square-mile iceberg broke off from the glacier. After studying satellite images before and after the event, researchers at Ohio State University found that in 2013, a rift formed at the base of the ice shelf, 20 miles inland. The rift worked its way up for two years until it caused the iceberg to break off.

Icebergs do separate from ice sheets in the Antarctic on a fairly regular basis. This one, though, is special. It confirmed what glaciologists have long been suspecting: that the ice shelf is weakening. But it also shows that the ice retreat is happening farther inland than scientists had previously observed.

“It’s generally accepted that it’s no longer a question of whether the West Antarctic Ice Sheet will melt, it’s a question of when,” study leader Ian Howat, associate professor of Earth sciences at Ohio State, said in a statement. If things continue the way they are, glaciers will keep melting, and West Antarctica will significantly collapse “in our lifetimes.”

In the case of the 2015 iceberg, researchers believe that the rift began deep down the ice shelf, where warming waters are eating away at the ice. That’s a new threat to the Antarctica ice sheet, where rifts usually form at the margins, not deep inland. Similar breakups had been observed in Greenland and the global consequences of melting ice in these regions are huge.

The Antarctic and Greenland ice sheets contain more than 99 percent of the freshwater ice on Earth. Previous papers have shown that the melting of the West Antarctic Ice Sheet is inevitable, and it could raise sea levels by as much as 10 feet. In the US, that would mean that cities like New York and Miami would go underwater.

Miami may be underwater by 2100