A 400-meter thick iceberg more than twice the size of Greater London is drifting northward between Elephant Island and the South Orkney islands off Antarctica as seen by an expedition ship.

The tooth-shaped iceberg named A23a, which is nearly 4,000 square kilometers across, had broken up from the Antarctic ocean floor where it had been stuck for three decades, Agence France-Presse reports.

It contains an estimated one trillion tons of fresh water that is likely to melt off into the ocean along way.

The ship run by the expeditions firm EYOS and led by Ian Strachan had been planning to go to South Georgia island but due to a bird flu outbreak there, it visited A23a instead.

A23a first broke off the Antarctic coast back in 1986, making it the world’s oldest iceberg, as well as its largest.

But it quickly became stuck to the ocean floor, where it languished for decades.

Whether or not its separation from the sea floor late last year was caused by climate change — winter Antarctic sea ice reached its lowest level on record last year — remains an open question.

Andrew Fleming of the British Antarctic Survey told AFP that one or two big icebergs break off every year.

“It’s more likely that it’s time had just come,” he added.

As the iceberg is “ejected out into the Southern Ocean,” warmer waters and bigger waves will start to break it up, Fleming said.

WITH AFP