A massive iceberg with enough freshwater in it to fill Sydney Harbour 135 times over is about to break off the Mertz glacier in Antarctica.

The iceberg will be 75 kilometres long and contains 750,000 gigalitres of ice.

Scientists are not sure if it is a natural event or if global warming is to blame. But a joint Australian and French team hope to find out.

The Mertz Glacier is near Commonwealth Bay in East Antarctica.

It flows into the southern ocean for 140 kilometres before it drops icebergs into the sea.

A large crack has formed about half way along the Mertz Glacier, which means it is going to drop a very large iceberg.

French glaciologist Benoit Legresy is measuring the break-up.

"Just at the moment, it's undergoing a massive calving event which promises to release an iceberg which will be between 20 to 25 kilometres wide and 75 kilometres long by about 400 to 500 metres thick," he said.

The iceberg contains enough fresh water to fill Sydney Harbour 135 times - that is 30 per cent of the world's annual water consumption.

When it breaks off, the iceberg won't melt straight away because it could take up to 30 years for the currents to move it to water that is warm enough to melt the ice.

The scientists realised the large iceberg was forming when they looked at satellite pictures of the Mertz Glacier and saw two large cracks. When the cracks kept getting bigger Benoit Legresy decided to measure the break-up.

Article continues: http://www.abc.net.au/news/stories/2009/05/08/2565209.htm?site=science&topic=latest