One of the world’s most enduring ice giants, known as A23a, is now collapsing rapidly after spending nearly 40 years in the ocean since detaching from Antarctica. The iceberg, once the largest on Earth, is now shrinking at a dramatic pace as it floats through warmer seas far from its icy origin.

“It’s basically rotting underneath,” said Andrew Meijers, a physical oceanographer from the British Antarctic Survey (BAS). “The water is way too warm for it to maintain. It’s constantly melting… I expect that to continue in the coming weeks, and expect it won’t be really identifiable within a few weeks.”

Once Immense, Now Crumbling

Earlier in 2025, A23a still spanned more than 3,600 square kilometers, tipping the scales at nearly a trillion tonnes an area larger than Rhode Island and more than double the size of Greater London. It had held the title of the largest current iceberg several times since it first broke off the Filchner-Ronne Ice Shelf in 1986.

Now, it has shrunk to approximately 1,700 square kilometers, according to satellite analysis from the EU Copernicus Earth observation program and BAS scientists. Despite being reduced to less than half its original size, the iceberg is still massive 60 kilometers across at its widest and its melting chunks pose navigational risks, with smaller fragments still large enough to threaten passing vessels.

Long Life, Unusual Journey

Unlike most icebergs that quickly disintegrate after calving, A23a remained grounded on the Weddell Sea seafloor for over 30 years before it became mobile in 2020, likely due to gradual melting that freed it from the ocean bottom. Since then, it has drifted along the powerful Antarctic Circumpolar Current, joining the well-known route referred to as “iceberg alley.”

The iceberg’s exceptional size and endurance have surprised scientists. “Most icebergs don’t make it this far. This one’s really big, so it has lasted longer and gone further than others,” Meijers said.

Grounding and Environmental Concerns

In March 2025, A23a temporarily ran aground near South Georgia Island, raising concerns for local wildlife. Scientists feared it could block access to feeding grounds for large populations of penguins and seals, potentially preventing adult animals from feeding their young. However, the iceberg dislodged in May, continued its northward drift, and has since picked up speed reportedly moving as much as 20 kilometers in a single day.

As A23a passed South Georgia, scientists aboard the RRS Sir David Attenborough visited the iceberg and collected water and seabed samples. According to BAS, “The grounding and enormous release of cold freshwater are likely to have had a major impact on organisms on the seabed and in the surrounding water.” The data is now being analysed in the UK to understand the iceberg’s ecological footprint.

Rapid Breakup Underway

Since June, massive pieces some over 400 square kilometers each have broken off A23a, accelerating its disintegration. “The iceberg is rapidly breaking up, and shedding very large chunks, themselves designated large icebergs by the US national ice centre that tracks these,” Meijers noted in a statement.

The iceberg is currently being steered by the Southern Antarctic Circumpolar Current Front (SACCF), which has been pulling it in an anti-clockwise loop around South Georgia. This powerful current is expected to carry A23a and its remaining fragments to the northeast, continuing along iceberg alley.

Changing Climate, Fragile Giants

Although iceberg calving the process of large ice masses breaking off ice shelves is a natural phenomenon, experts say its frequency and severity are increasing, likely due to human-induced climate change. “Ice shelves have lost trillions of tons of ice through increased iceberg formation and melting over the past few decades,” Meijers said, linking the trend to warming seas and shifts in ocean currents.

Still, he cautioned that the rarity of such massive bergs makes it difficult to draw statistical conclusions. Yet the breakup of A23a and similar giants, such as A68 in 2021 and A76 in 2023, suggests a pattern worth monitoring as the planet warms.

A New Iceberg King

With A23a disintegrating rapidly, the mantle of the world’s largest iceberg has passed to D15a, which is currently stationary near Australia’s Davis research base, covering around 3,000 square kilometers. Meanwhile, A23a is expected to fracture into pieces too small to track in the coming weeks, finally ending a journey that began nearly four decades ago.