One of the largest icebergs ever recorded has broken away from Antarctica’s Larsen C ice shelf.

The 5,800 square-kilometre chunk of ice had been hovering near the breaking point since the spring, when rapid advancement of a crack in the shelf left the iceberg hanging on by a thread of just four and a half kilometres of ice. The final break was identified by NASA’s Aqua MODIS satellite and is believed to have happened sometime between July 10 and July 12.

“We have been anticipating this event for months, and have been surprised by how long it took for the rift to break through the final few kilometres of ice,” Adrian Luckman, lead researcher with the U.K.-based Project MIDAS, wrote on the project’s blog. “We will continue to monitor both the impact of this calving event on the Larsen C Ice Shelf, and the fate of this huge iceberg.” 

Project MIDAS has been monitoring the rift in the Larsen C ice shelf since a similar rift caused the sudden collapse of the Larsen B ice shelf in 2002. 

Ice shelves are floating masses of glacial ice that over time fill entire bays and fjords and can grow to be hundreds of metres thick; Larsen C is between 200 and 600 metres thick. They’re important because they act as a buffer for the glaciers behind them, protecting the glaciers from erosion and preventing the rapid flow of ice into the sea. 

Adrienne White, a glaciologist with the University of Ottawa’s Laboratory for Cryospheric Research who studies ice shelves in the Canadian High Arctic but has been on several expeditions to Antarctica, likens the effect to a cork in a wine bottle. “If you take away the cork, the wine flows; if you take away the ice, the glacier will flow. Without the ice shelf, the glacier is exposed to wind, waves, tides and the impact of sea ice and icebergs colliding with the coastline,” she explains. 

Iceberg calving is a natural part of the life cycle of an ice shelf, but an event of this size has the potential to destabilize the entire shelf. If that buffer is lost entirely, it would allow more glacial ice to enter the ocean, which could eventually have an impact on global sea levels. 

It’s unclear how this massive calving event will affect the Larsen C ice shelf in the long run; it could start to regrow, but it could also become increasingly unstable over the next several years and eventually collapse. Whatever the fate of Larsen C, White says of more immediate concern to scientists is what will happen to the massive ice island that has broken away. 

“You’ve now got this trillion-tonne iceberg floating around, and we don’t know if it will stay intact, or break into a couple of smaller pieces, or break into hundreds of smaller pieces. If it starts to float and break up it could disrupt ship traffic and wildlife migration patterns.”