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.

McDonald Ice Rumples Brunt Ice Shelf FEB 7, 2019 OCT 2018 DEC 2017 Chasm 1 MAR 2017 3 mi 3 km JOHN KAPPLER, NG STAFF SOURCE: COPERNICUS SENTINEL DATA, 2019 McDonald Ice Rumples Brunt Ice Shelf February 7, 2019 October 2018 December 2017 March 2017 3 mi 3 km JOHN KAPPLER, NG STAFF SOURCE: COPERNICUS SENTINEL DATA, 2019

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 Research Station, a major center for the British Antarctic Survey’s research activities in the region. But no one knows exactly when the berg will dislodge.

Nor is the berg the biggest to break off Antarctic ice sheets in recent years. That prize goes to a Delaware-sized chunk (something like 2,240 square miles) that peeled off the nearby Larsen-C ice sheet in 2017. But this hunk is nothing to sneeze at.

Unlike the dramatic ice break-ups observed over the past few years on the Antarctic peninsula, though, this calving event is not likely driven by warming air or seas.

“The Brunt ice shelf is relatively far south compared to the ice shelves that have calved dramatically on the Antarctic Peninsula,” says Oliver Marsh, a glaciologist at the British Antarctic Survey in Cambridge, England, and the extra-warm air temperatures that have toasted that region haven’t had such a dramatic effect at Brunt yet. “This calving event is just part of a natural cycle.”

The first fault line on the Brunt Ice Shelf, called Chasm 1, has been in place for over 35 years (glaciologists call breaks in the ice sheet that extend all the way from surface to the ocean below “chasms,” while shallower faults are called “cracks” or “crevasses”). It remained relatively stable until 2012, when surveyors noticed that it starting to grow again, creeping across the wide tongue of ice that protruded out over the ocean. Now, it’s extending outward in fits and starts, growing by a few hundred meters each week, on average.

60°W 50° 40° Atlantic Ocean MAP AREA 60°S South Shetland Is. ANTARCTICA ANTARCTIC PENINSULA Larsen Ice LOCATION OF CRACKING Shelf Neumeyer III Germany Weddell Sea Riiser-Larsen Ice Shelf Brunt Ice Shelf Halley U.K. SANAE IV S. Africa Ronne Ice Shelf ELLSWORTH LAND COATS LAND Research Station Vinson Massif 500 mi 16,067 ft 500 km 80° JOHN KAPPLER, NG STAFF 60° 50° 40° 30° SOUTH SHETLAND IS. Atlantic Ocean MAP AREA 20° ANTARCTICA 70°W ANTARCTIC PENINSULA Larsen Ice Shelf Alexander Island LOCATION OF CRACKING Neumeyer III Germany Riiser-Larsen Ice Shelf Weddell Sea 70°S 70° Seal Bay Lyddan Brunt Ice Shelf Island Aboa Finland Halley U.K. SANAE IV S. Africa Svea Sweden Ronne Ice Shelf Ellsworth land Belgrano II Argentina COATS LAND Kohnen Germany Filchner Vinson Massif Ice Shelf 16,067 ft Research Station 250 mi ANTARCTICA 250 km JOHN KAPPLER, NG STAFF 80° 80°

The second break point, near the top of the image and first observed in 2016, is known as the Halloween crack. The crack is upstream from a set of crinkles, visible from the satellite photos, called the McDonald Ice Rumples. The rumples form when the bottom of the ice shelf, which flows downhill toward the ocean like very slow putty, runs up against an underwater ridge. That ridge pins the ice in place and slows its flow, forcing ice upstream to pile up, fold, crinkle, and sometimes crack.