A city-sized Antarctic iceberg known as A23a has formed a vast pool of meltwater on its surface, raising concerns it may soon break apart. Satellite imagery reveals an unusual raised ice rim holding billions of litres of water, resembling an oversized swimming pool. Scientists suggest this pooling could accelerate the berg's fragmentation in warmer waters.
The tabular iceberg A23a, which calved from Antarctica's Filchner-Ronne ice shelf in 1986, was once more than five times its current size and held the record as the world's largest. For decades it remained grounded, but in recent years it has drifted northward into the Southern Ocean's warmer waters and air, leading to ongoing fragmentation.
Satellite photos now show meltwater collecting in an extraordinary manner on A23a's surface. A raised rim of ice encircles the entire cliff edge, spanning about 800 square kilometres—an area larger than Chicago—and trapping the water like a giant children's play pool. In some spots, the ponded water appears a deep, vivid blue, indicating depths of several metres. The total volume likely amounts to billions of litres, sufficient to fill thousands of Olympic-sized swimming pools.
Douglas MacAyeal at the University of Chicago explains the phenomenon: “My theory is that the edges are bent, nose-down, creating an arch-like dam on the top surface that keeps the meltwater inside.” He attributes this bending to undercutting by waves and melting, combined with the natural tendency of ice cliffs to curve even if initially vertical. The visible streaks of surface water trace the berg's original flow patterns from when it was part of the Antarctic coastline.
Experts warn that this meltwater accumulation could hasten A23a's breakup. Mike Meredith at the British Antarctic Survey notes: “If that water drains into cracks and refreezes, it will prise the berg open.” He adds that the iceberg could disintegrate into mush almost overnight, highlighting the dynamic processes at play in polar regions.
