City-sized iceberg has turned into a giant swimming pool

Meltwater from a city-sized iceberg in the Southern Ocean is quickly forming a giant puddle on its surface – perhaps a sign that it is close to breaking up.

Scientists are fascinated by the frozen colossus, known as A23a, because meltwater collects and is held on its surface in an unusual way.

Satellite images show a raised edge of ice encircling the entire rock edge of the tabular Antarctic iceberg, giving it the appearance of a huge children's play pool, except it is about 800 square kilometers in area, an area larger than Chicago.

In some places the water in the pond appears deep and bright blue, suggesting a depth of several meters. Throughout the A23a, the volume of water is likely to be in the billions of liters – enough to fill thousands of Olympic-sized swimming pools.

Douglas McEil The University of Chicago says the edge effect is typical of the world's largest icebergs.

“My theory is that the edges are bent and point downward, creating an arched dam on the top surface that keeps meltwater inside,” he says. “The bending is likely a combination of wave undercutting and melting, as well as the natural tendency of ice cliffs to tilt even when they would otherwise be perfectly vertical.”

The streaks of surface water visible in satellite images are a relic of how ice once flowed when the iceberg was still attached to the Antarctic coastline, he said.

A23a is an old iceberg. It separated from the Filchner-Ronne Ice Shelf in 1986 and was then more than five times its current size. For a while, it held the title of the largest iceberg in the world.

However, in recent years it has moved north into warmer waters and air and is now undergoing inexorable fragmentation. The huge volume of melt water accumulating on its surface can completely destroy it. “If this water flows into the cracks and freezes again, the iceberg will open,” says Mike Meredith at the British Antarctic Survey.

It can turn to mush almost overnight, he said.