By Jonathan Amos, BBC Science Correspondent

The world's biggest iceberg which broke free from the Antarctic in 2017 is about to enter the open ocean where its size is expected to be dramatically reduced as it hits the Southern Ocean.

Photo: NASA / John Sonntag

A68 has pushed so far north it is now at the limit of the continent's perennial sea-ice.

When it calved, the berg had an area close to 6000 sqkm and it has lost very little of its bulk over the past two-and-a-half years.

But scientists say the colossus will struggle to maintain its integrity when it reaches the Southern Ocean's rougher waters.

"With a thickness to length ratio akin to five sheets of A4, I am astonished that the ocean waves haven't already made ice cubes out of A68," Professor Adrian Luckman from Swansea University in the UK said.

"If it survives for long as one piece when it moves beyond the edge of the sea-ice, I will be very surprised," he told BBC News.

We stopped by in the Weddell Sea to visit #A68 #IcebergA68 - Stunning and spectacular! Wonder how long it may last? pic.twitter.com/MifQ2tJVCF — G Expedition (@g_expedition) December 12, 2019

A68 split from the Larsen C Ice Shelf in July 2017. For a year, it hardly moved, its keel apparently grounded on the seafloor.

Photo: Copernicus Data/Sentinel-3/ESA/A.Luckman

But the prevailing winds and currents eventually began to push it northwards along the eastern coast of the Antarctic Peninsula, and during this summer season the drift has undergone a rapid acceleration.

The iceberg, currently at 63 degrees South latitude, is following a very predictable course.

When it pops above the tip of the peninsula, the massive block should be swept northwards towards the Atlantic - a path researchers refer to as "Iceberg Alley".

Many of Antarctica's greatest bergs even make it as far - and beyond - the British Overseas Territory of South Georgia at roughly 54 degrees South.

The biggest ever recorded iceberg in the modern era was the 11,000-sqkm block called B15, which calved from the Ross Ice Shelf in 2000.

One of its last remnants, now measuring "just" 200 sqkm, is halfway to the South Sandwich Islands, east of South Georgia.

Objects this big have to be constantly monitored via satellites because they pose a risk to shipping.

While they keep an eye on A68, scientists are also watching for two other, soon-to-birth bergs.

One is about to come off the front of Pine Island Glacier in the West Antarctic. This will be a little over 300 sqkm when it calves. The block is already riven with many cracks.

"I expect that the new iceberg will break into many pieces soon after it calves," Luckman said.

The other imminent large berg is forming in eastern Antarctica, on the edge of the Brunt Ice Shelf.

This should be about 1500 sqkm - roughly the area of Greater London.

The putative berg has garnered a lot of attention because Britain's Halley research station had to be moved to make sure it wasn't in harm's way.

The berg will calve when a big rift, dubbed Chasm 1, finally slices through a section of ice measuring less than 10km in length.

Precisely when, no-one can say. "The rift is widening, but only at a steady rate, and the rift tip is hardly advancing," Luckman said.

- BBC