Antarctica has shed two new, large icebergs into the Southern Ocean. The bergs are the result of a crack that had been spreading across the Nansen Ice Shelf.

The progression of the crack was visible in a pair of satellite images acquired in December 2013 and 2015. Ryan Walker and Christine Dow, glaciologists at NASA’s Goddard Space Flight Center, flew along the crack in late 2015. It was clearly still attached. On April 6, 2016, with southern winter soon to set in, satellite imagery indicated that the cracking ice front was still holding on.

Then on April 7, 2016, in the last days before winter darkness, the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite acquired this image as the bergs broke away.

The Nansen Ice Shelf previously measured about 35 kilometers (20 miles) across and 50 kilometers (30 miles) long. For comparison, the Drygalski Ice Tongue just south of Nansen stretches 80 kilometers (50 miles) into the sea. Of the two bergs shed from Nansen, only one is large enough to meet the size criteria for naming and tracking by the U.S. National Ice Center. This larger piece is named C33.

But why did the crack finally give out? According to Walker, summer melting probably helped weaken and break up the shelf fragments and sea ice (the mélange) within the crack, which acted like glue to keep the bergs attached. Summer melt also could have helped the deeply fissured ice to break further, completing the crack across the shelf.

Once broken off, the new icebergs would have been blown away from the shelf by the strong katabatic winds that blow out to sea. “Nansen usually has pretty strong katabatic winds,” Walker said.

Walker emphasized that this is routine iceberg calving—there are indications that similar events occurred there in the 1960s—and not a collapse of the ice shelf. Still, some scientists are concerned for a different reason; the icebergs are threatening scientific equipment in the area. Scientists at New Zealand’s National Institute of Water and Atmospheric Research (NIWA) say the bergs are deep enough that they cold snag a mooring deployed in Terra Nova Bay. The mooring collects data on the effects of climate change on sea ice and ice shelves.

“We won’t know until we go back next summer whether it is still there. We could lose a whole year of data. If that happens it will leave a gap in our research and that’s unfortunate,” said oceanographer Mike Williams in a NIWA press release. “However, it is a risk we have to take. We could see the crack from satellite images but predicting when an ice shelf will calve is difficult. It could have happened any time in the next five years.”

NASA Earth Observatory image by Jesse Allen, using data from the Land Atmosphere Near real-time Capability for EOS (LANCE). Caption by Kathryn Hansen.