For several months now, scientists have been monitoring growing cracks in Antarctica’s Brunt Ice Shelf which have been spreading at an alarming rate. At almost any time, perhaps even in days, an iceberg over 560 square miles, or twice the size of New York City, is expected to calve from the ice shelf.

While the crack was first noted 35 years ago, it was slow moving and the ice shelf has been stable. Recently, however, the crack has started accelerating toward another fissure called the Halloween crack.

“We don’t have a clear picture of what drives the shelf’s periods of advance and retreat through calving,” Chris Shuman, a glaciologist with NASA and the University of Maryland, Baltimore County, said in a statement quoted by USA Today. “The likely future loss of the ice on the other side of the Halloween Crack suggests that more instability is possible.” 

The iceberg itself isn’t as big as other recent bergs. In 2017, an iceberg the size of Delaware broke off the Antarctic ice shelf, one of the largest on record. The potential iceberg, however, would be the largest to break from the Brunt Ice Shelf since observations started in 1915.

It’s not clear when the iceberg will break off, Shuman said, but researchers are closely watching it. “It could calve within the next few days, or weeks, or possibly even longer.”