This iceberg was once the biggest in the world. Now it has just weeks left

The story of one of the world’s oldest icebergs is nearly at an end, after a breathtaking 40-year journey that has captivated scientists.

The iceberg, known as A23a, was once the largest on Earth, covering an area more than twice the size of Greater London.

But after a path full of twists and turns, A23a has melted, fractured and spectacularly disintegrated over the past year.

Now, far from the icy seas of Antarctica, what’s left of A23a is being eaten away by warmer waters. It’s in its death throes, not expected to last more than a matter of weeks.

All icebergs melt eventually, but scientists have been looking at how it's disintegrated for clues about how other parts of Antarctica might respond as the climate changes.

“It’s been an extraordinary journey,” says Prof Mike Meredith of the British Antarctic Survey in Cambridge. “But it is on its last legs now.”

This is the story of A23a’s final months.

But first we have to go back to 1986. That year, a nuclear reactor exploded at the Chernobyl power plant in what is now northern Ukraine, Gary Lineker won the golden boot at the Fifa World Cup in Mexico, and Whitney Houston received her first Grammy award.

Away from the world’s gaze, the Filchner Ice Shelf - a massive floating tongue of ice extending from the Antarctic continent and into the Weddell Sea - was changing dramatically. One of the icebergs to break off - or calve - was A23a, then about 4,000 sq km (roughly 1,540 sq miles).

It soon became anchored in the muds of the Weddell Sea, where it remained stuck for more than 30 years. It wasn’t until 2020 that scientists noticed signs that A23a was on the move again.

While it’s likely icebergs have lived longer in the Earth’s distant past, A23a is thought to be the oldest iceberg in the world today, at least among those picked up by satellites and tracked by scientists.

“Its journey is really pretty impressive, just for sheer longevity,” says Dr Christopher Shuman, a retired scientist formerly with the University of Maryland, Baltimore County, in the US. He likens tracking its path to watching a TV drama “where you don't know what you're going to see next”.

As A23a moves across the vast South Atlantic Ocean, it can be hard to grasp its scale - but if you could drop it into the English Channel its size would be much more striking.

At the start of 2025 - even after 39 years - A23a was still a collosus. It would have almost stretched between the Isle of Wight and Normandy in France. Now, it wouldn’t even reach halfway from Dover to Calais.

“To watch it be so stable for so long, and then just disintegrate over one year, has been fascinating,” says Dr Catherine Walker of the Woods Hole Oceanographic Institution in the US, who was born the same year as A23a.

Through the first half of 2025, A23a shrank by about a quarter, as chunks of ice broke off and ocean waters ate away at its sides and base. By midway through the year, A23a had lost its title as the world’s biggest iceberg, but it was still going strong.

Then in August and September, A23a found itself in an area of the South Atlantic Ocean above the North-west Georgia Rise. It’s a mound on the seabed roughly a couple of kilometres high, about 1,500km (930 miles) east of the Falklands.

Above the rise, A23a appeared to be spinning on a rotating column of ocean water for a few weeks. Scientists believe those mechanical forces on an iceberg already weakened by warm waters could have helped to effectively pull the berg apart.

Several large icebergs broke off A23a in quick succession - bergs big enough to be given their own names, A23g, A23h and A23i, showing they calved from the original.

In late December, at the height of the Southern Hemisphere summer, the iceberg came under attack from warm air too. Vivid blue meltwater - perhaps several metres or more deep - appeared across its surface, trapped in place by the berg’s rim-like edges known as ramparts.

“It was beautiful to look at, but a real sign that it was melting from above as well as from below,” says Meredith. “At that point, we realised this iceberg was getting pretty mushy and wasn’t going to live long.”

“When you have this heavy water sitting on the top of an iceberg, it wants to go downward,” adds Walker. “And so, once you allow that water to percolate through cracks, it'll spread those cracks out, and you get these fracturing or calving events.”

This fracturing of water-filled cracks - known as “hydrofracture” - appears to be exactly what happened in late December and early January.

The mixture or “mélange” of small pieces of ice next to the main berg is evidence of what Shuman describes as a “blowout”, triggered by the drainage of meltwater from the iceberg.

The calving - and melting - of the largest icebergs like A23a is a natural phenomenon and not necessarily the result of climate change, although parts of Antarctica are now shedding icebergs faster than the ice can be replaced.

Scientists have been closely watching A23a’s disintegration for hints as to how Antarctica might respond to rising temperatures - specifically its ice shelves, the floating tongues of glaciers that extend into the ocean.

Ice shelves play an important role in the stability of much of the Antarctic ice sheet. But it’s by no means clear how quickly they might collapse in a warming climate and what that could mean for sea-level rise.

While not an exact match, icebergs can act as “travelling natural laboratories” for how some of these processes could unfold, explains Walker.

“We can learn a lot from how these big icebergs evolve in warmer conditions, and then try to map that onto what we expect ice shelves will do,” she says.

In the 11 days up to 22 February, the now smaller, lighter berg travelled more than 700km (435 miles) north-east across the South Atlantic, hurtling along at an average of about 2.7km/h (1.7mph).

That journey exposed A23a to warmer waters, close to 10C at the surface - bad news for an iceberg.

“Every day, all day, it's in warmer and warmer water,” says Shuman. “It’s just like ice in your drink. It doesn't take all that long to disappear.”

Over the past two weeks A23a has been carried by ocean currents in a near-complete clockwise loop. This could be its final dance.

Recent satellite images suggest further hydrofracturing of what was left of it - “tantalising evidence of sudden disintegration”, according to Prof Adrian Luckman of Swansea University.

While other icebergs have travelled further in the past, A23a is the furthest north of any Antarctic iceberg being tracked by scientists today. It’s closer to the equator than London.

The prolonged exposure to sea warmth means the berg’s remains will inevitably fragment and eventually melt away, even though the Southern Hemisphere winter is on the horizon.

By 5 March, A23a had shrunk to approximately 180 sq km (70 sq miles), although estimates can vary slightly.

Once it gets to roughly 70 sq km (27 sq miles), scientists will stop tracking it. That moment’s not far away, according to Luckman.

"All traces will probably have disappeared in a matter of weeks now, at most."