At the bottom of the planet, a huge iceberg about twice the size of New York City is about to break away from Antarctica's Brunt Ice Shelf.

“We’re keeping a good eye on it,” said Dominic Hodgson, a senior scientist at the British Antarctic Survey in Cambridge, England, who has been using satellite imagery to watch the ice shelf. “It’s one of the most intensely monitored ice shelves in Antarctica. It’s quite interesting — it’s like a real-time science experiment.”

Scientists are watching the progress of a huge crack in the ice shelf that began growing recently after being stable for more then three decades. Dubbed Chasm 1, the rift has grown by 200 meters (more than 650 feet) in the past month, extending northward toward another big rift known as the Halloween Crack not far from a research facility maintained by the Survey.

The converging cracks mean that the separation, or calving, of an iceberg covering about 660 square miles is “geologically imminent,” Hodgson said. But, he added, “whether that means it happens next week or in a month's time, we really don’t know.”