Scientists monitoring a rift in an Antarctic ice shelf where an iceberg a quarter the size of Wales is poised to break off say the huge crack in the ice has spread.
Late last year a rift in the Larsen C Ice Shelf grew suddenly by around 18km, leaving a vast iceberg more than 5,000 square kilometres "hanging by a thread".
Just 20km of ice connects the iceberg to the rest of the ice shelf, according to researchers from the Swansea University-led Midas project, which has been studying the stability of the Larsen C Ice Shelf for three years.
The main rift continued to grow early this year and is currently 180km long.
Now satellite data has revealed a second branch of the rift, some 15km long, which is moving towards the edge of the ice.
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When the ice breaks off it is likely to lead to one of the biggest icebergs ever recorded.
Professor Adrian Luckman, of Swansea University College of Science and head of Project Midas, said: "While the previous rift tip has not advanced, a new branch of the rift has been initiated.
"This is approximately 10km behind the previous tip, heading towards the ice-front."
He said it was the first significant change to the rift since February, but added: "Although the rift length has been static for several months, it has been steadily widening, at rates in excess of a metre per day."
And he said: "When it calves, the Larsen C Ice Shelf will lose more than 10 per cent of its area to leave the ice front at its most retreated position ever recorded; this event will fundamentally change the landscape of the Antarctic Peninsula."
The researchers warned the ice shelf will be less stable after the iceberg calves, and could follow the example of its neighbouring ice shelf Larsen B, which disintegrated in 2002 after a similar event.
