The world’s biggest iceberg broke free and is drifting again after decades stuck

For most of the last four decades, the largest single block of floating ice on the planet sat almost exactly where it broke off, pinned to the seafloor of the Weddell Sea like a runaway that never quite got away. Then, within the space of a few years, it worked itself loose, spun in place for months, and finally began a long drift north toward warmer water.

A slab that calved during the Cold War

The iceberg, known by the catalog designation A23a, broke away from the Filchner-Ronne Ice Shelf in Antarctica back in 1986. A Soviet research station called Druzhnaya-1 had been built directly on the ice shelf where the berg formed, and it ended up sitting on the newly calved slab when the shelf gave way. A rescue mission the following year relocated the station’s equipment before the ice could carry it out to sea, an early sign of just how enormous the new iceberg was going to be.

According to the tracking record maintained on Wikipedia’s entry for A23a, the berg ran aground on the floor of the Weddell Sea rather than drifting off immediately, and it stayed there for roughly three decades, effectively frozen in place even as smaller icebergs around it came and went. It was not until 2020 that satellite trackers first noticed the giant slab beginning to shift.

Freed from the seabed, then trapped in a spinning vortex

By November 2023, A23a had moved far enough to be tracked passing the northern tip of the Antarctic Peninsula, and the following month the polar research ship RRS Sir David Attenborough intercepted it to sample the mineral dust the melting ice was releasing into the surrounding water. Even at a cruising speed of 10 knots, the ship needed several hours just to sail alongside two sides of the berg, a rough sense of scale for an object that was still around 3,500 square kilometers in area at the time.

The iceberg’s escape from the seabed did not mean a straight run to open ocean. In early 2024 it entered the Antarctic Circumpolar Current but stopped making forward progress, and by August scientists confirmed it had been caught in what is known as a Taylor column, a rotating water formation above an underwater seamount near the South Orkney Islands. A23a spun in place there for months, turning roughly 15 degrees a day, before the British Antarctic Survey reported in December 2024 that it had finally broken out of the trap and resumed drifting north.

A near miss with a penguin colony’s feeding grounds

The current the iceberg rode next carried it toward South Georgia, a remote island in the South Atlantic that hosts enormous colonies of king penguins along with millions of elephant and fur seals. Scientists warned that if the berg grounded directly against the island’s shallow continental shelf, it could block the narrow channels those animals rely on to reach their feeding grounds, echoing an earlier iceberg grounding near the island in 2004 that was blamed for stranding young seals and penguins.

In early March 2025, A23a struck the shelf and came to a stop roughly 45 miles from South Georgia rather than colliding with the island directly. Oceanographers monitoring the grounding said that if the berg stayed fixed in that position, the disruption to local wildlife would likely be limited, and some researchers noted the opposite effect was also possible: a grounded iceberg can stir up seafloor nutrients in ways that temporarily boost the surrounding marine ecosystem.

Breaking apart in warmer water

The grounding did not hold for long. By July 2025, A23a had begun drifting again, moving east and north of South Georgia, and by early September satellite imagery analyzed by Agence France-Presse showed the berg actively breaking apart, having lost more than half its original area to roughly 1,770 square kilometers. Chunks as large as 400 square kilometers calved off in the process, and smaller fragments, some still large enough to threaten passing ships, scattered across the surrounding sea.

The disintegration accelerated from there. By November 2025, the main remaining mass had shrunk to around 1,300 square kilometers, with a separate 300-square-kilometer fragment tracked farther north. NASA reported in January 2026 that meltwater pooling on the berg’s surface had turned it visibly blue, a sign it was on the verge of falling apart entirely as ocean currents pushed it into progressively warmer water.

Watching a 40-year-old giant disappear

The iceberg’s slow collapse has become one of the most closely tracked events of its kind, in part because so few bergs of its size survive long enough to be observed calving, grounding, spinning and finally dissolving over the course of several years. Researchers see the episode as a useful natural experiment on how Antarctic ice behaves once it leaves the continent, even though A23a’s original break from the shelf predates the recent run of climate-linked ice loss and cannot itself be attributed to a warming ocean. By March 2026, its area had fallen to roughly 180 square kilometers, and trackers noted that once a berg shrinks to about 70 square kilometers, it typically becomes too small and fragmented to follow by satellite at all.

This article was produced with the assistance of AI and reviewed by an editor.

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