One of the world’s oldest and once-largest icebergs, known as A23a, is approaching the end of its remarkable four-decade-long journey after gradually breaking apart and melting in warmer waters of the South Atlantic Ocean. Scientists who have tracked the massive ice formation for years say the iceberg is now in its final phase and is unlikely to survive for more than a few weeks.
A23a, which once held the distinction of being the largest iceberg on Earth, originally covered an area of about 4,000 square kilometres—more than twice the size of Greater London. Over the past year, however, the colossal block of ice has shrunk dramatically as it fractured, calved into smaller icebergs and slowly melted away. Researchers monitoring its movement say the remnants of the iceberg are now far from Antarctica and are rapidly disintegrating in relatively warmer ocean waters.
Experts say the collapse of A23a is not unusual in itself, as all icebergs eventually melt. However, the iceberg’s unusual longevity and dramatic breakup have made it an important subject of study for scientists trying to understand how Antarctic ice formations might respond to a warming climate.
“It’s been an extraordinary journey,” said Professor Mike Meredith of the British Antarctic Survey in Cambridge. “But it is on its last legs now.”
The iceberg’s story began nearly four decades ago. In 1986, when global headlines were dominated by events such as the Chernobyl nuclear disaster and the FIFA World Cup in Mexico, a massive chunk of ice broke away from the Filchner Ice Shelf in Antarctica. The ice shelf, a vast floating extension of the Antarctic continent that stretches into the Weddell Sea, periodically releases large icebergs in a process known as calving. One of the icebergs produced during that event was A23a.
Shortly after breaking away, the iceberg became grounded on the seabed in the Weddell Sea, where it remained stuck for more than 30 years. It was not until 2020 that scientists detected signs that A23a had finally begun moving again, drifting slowly through the Southern Ocean.
Among icebergs currently monitored by satellites and researchers, A23a is believed to have been the oldest still in existence. Its prolonged survival and unpredictable movement have fascinated scientists studying polar ice dynamics.
Dr Christopher Shuman, a retired scientist formerly with the University of Maryland, Baltimore County, described tracking the iceberg as similar to watching a suspenseful television drama. According to him, the iceberg’s path has been unpredictable, with each phase revealing new developments.
Even after nearly four decades, A23a remained an enormous structure at the start of 2025. At that time, the iceberg was so large that it could have almost stretched across the English Channel between the Isle of Wight and the French coast near Normandy. Today, however, the remaining fragments would not even reach halfway across the channel between Dover and Calais.
Scientists say the iceberg’s rapid decline over the past year has been striking. Dr Catherine Walker of the Woods Hole Oceanographic Institution in the United States noted that A23a had remained stable for decades before undergoing rapid fragmentation within a short period.
During the first half of 2025 alone, the iceberg lost roughly a quarter of its mass as large chunks broke off and ocean water eroded its sides and base. Although it eventually lost its status as the world’s largest iceberg, it continued drifting across the South Atlantic.
A turning point came in August and September when A23a reached waters above the North-west Georgia Rise, an underwater elevation about 1,500 kilometres east of the Falkland Islands. The seabed feature rises roughly two kilometres high and influences ocean currents in the region.
While passing over the area, the iceberg appeared to spin slowly in place for several weeks, likely caught in a rotating column of ocean water. Scientists believe these mechanical forces placed additional stress on the already weakened iceberg, contributing to its eventual breakup.
Soon afterward, several large fragments broke away from A23a. These newly formed icebergs were large enough to receive their own designations—A23g, A23h and A23i—indicating that they originated from the original iceberg.
By late December, the iceberg was also exposed to warmer air temperatures during the peak of the Southern Hemisphere summer. Pools of bright blue meltwater began forming on its surface, sometimes several metres deep. These pools were trapped by natural rim-like structures known as ramparts that surround the iceberg’s surface.
Although visually striking, the meltwater was a clear sign that the iceberg was weakening rapidly. According to researchers, the presence of large volumes of water on the surface can accelerate structural damage.
“When you have heavy meltwater sitting on top of an iceberg, it tends to force its way downward,” Walker explained. As the water seeps into cracks within the ice, it expands them, eventually causing fractures and large break-offs.
This process, known as hydrofracture, appears to have played a major role in A23a’s final disintegration in late December and early January. Satellite images showed a mixture of broken ice pieces surrounding the main iceberg, suggesting what scientists describe as a dramatic “blowout” event triggered by the sudden drainage of surface meltwater.
With its structure now severely weakened and its fragments continuing to erode, researchers say the once-mighty iceberg that spent decades drifting across the Southern Ocean is now entering its final days.
