A chunk of ice more than twice the size of New York City could break off from Antarctica’s Brunt Ice shelf essentially at any moment.

Two large cracks in the shelf have been inching further open over the past few years. When they finally intersect, a berg about 660 square miles wide and almost 500 feet thick will be released into the ocean—where it will eventually melt, joining its brethren in adding more water to the world’s oceans and pushing global sea levels a tiny bit higher.

It’s no surprise that the giant berg would eventually break off: The Brunt Ice Shelf is one of the most carefully monitored ice tongues in the world, because it’s the site of the Halley