The world’s oldest and largest iceberg is refusing to melt, say scientists.

A few months ago, the 1,500-square-mile floating mass of ice known as A23a was expected to drift to warmer waters and eventually dissolve.

But the trillion-tonne iceberg, twice the size of Greater London and three times that of New York City, is instead stuck in an ocean vortex that could keep it in the same spot for years.

A23a, which once hosted a Soviet research station, broke away from the Antarctic coast in 1986. Almost immediately, it grounded on the seabed and was stuck for more than three decades. In 2020, it refloated.

Late last year, it began migrating, exciting scientists who said it was rare to see an iceberg of such size on the move. Helped by strong winds and currents, it moved out of the Weddell Sea into the Southern Ocean, drifting – around walking pace – towards warmer waters.

In April, it entered a powerful ocean current, predicted to funnel it into the South Atlantic where it would break up. But, unexpectedly, it has stopped.

“Usually you think of icebergs as being transient things; they fragment and melt away. But not this one,” Prof Mark Brandon, a polar expert, told BBC News. “A23a is the iceberg that just refuses to die.”

The huge berg is now slowly spinning just north of the South Orkney Islands, a barren part of the British Antarctic Territory uninhabited except for an Antarctic exploration base.

The iceberg has stopped not because it has hit the seafloor, but because it is trapped in a vortex caused by the Pirie Bank, a bump on the ocean floor. As the current meets that obstruction, it separates into two flows, producing a rotating swirl of water in between.

“The ocean is full of surprises, and this dynamical feature is one of the cutest you’ll ever see,” Prof Mike Meredith from the British Antarctic Survey told BBC News.

A23a, which weighs nearly one trillion metric tonnes, could be stuck for years, scientists say.

The iceberg’s continued survival comes as Antarctica loses ice, adding to rising global sea levels.

Last year, winter Antarctic sea ice fell to its lowest level on record. There were more than two million square kilometres (800,000 square miles) less ice than usual, an area ten times the size of the UK, according to the British Antarctic Survey.

It said such a low level of ice was “extremely unlikely to happen without the influence of climate change”. Persistent low sea ice could have profound impacts on weather systems and Southern Ocean ecosystems, including whales and penguins.