The world’s largest and oldest iceberg, a ‘mega-berg’ double the size of Greater London and weighing more than one trillion tonnes, has made a break for it.

After decades being grounded on the seafloor and more recently, spinning on the spot, this mega-iceberg has now broken free from its position north of the South Orkney Islands and is now drifting in the Southern Ocean.

The colossus iceberg, named A23a is twice the size of Greater London. It also weighs a whopping one trillion tonnes. It was calved from Antarctica’s Filchner Ice Shelf in 1986 and remained grounded on the seabed of the Weddell Sea for over 30 years, before beginning its slow journey north in 2020.

It’s a journey that has been marked by a series of intriguing scientific events. For months, the iceberg was trapped in a Taylor Column, an oceanographic phenomenon in which rotating water above a seamount traps objects in place.

It was this dynamic that kept A23a spinning in one spot, delaying its expected rapid drift north.

Having now made a break for it, it’s anticipated A23a will continue its journey into the Southern Ocean following the Antarctic Circumpolar Current, which scientists at the British Antarctic Survey believe will drive it towards the sub-Antarctic island of South Georgia. Once in this region, it will encounter warmer water where it will be expected to break up into smaller icebergs and eventually melt.

“It’s exciting to see A23a on the move again after periods of being stuck,” said Dr Andrew Meijers, an oceanographer at British Antarctic Survey, who has co-led the Ocean:Ice project to better understand how ice sheets affect the ocean.

“We are interested to see if it takes the same route the other large icebergs that have calved off Antarctica have taken. And, more importantly, what impacts this will have on the local ecosystem.”

Exactly one year ago, researchers aboard the British Antarctic Survey’s research vessel, the RRS Sir David Attenborough, observed and studied the iceberg as they were on a science mission in the Weddell Sea for the Biopole project. Scientists took the first photos of the moving iceberg while looking at how Antarctic ecosystems and sea ice influence global ocean cycles of carbon and nutrients, collecting data from alongside the cast iceberg.

Laura Taylor, a biogeochemist on the Biopole cruise, said: “We know these giant icebergs can provide nutrients to the waters they pass through, creating thriving ecosystems in otherwise less productive areas. What we don’t know is what difference particular icebergs, their scale, and their origins can make to that process.

“We took samples of ocean surface waters behind, immediately adjacent to, and ahead of the iceberg’s route. They should help us determine what life could form around A23a and how it impacts carbon in the ocean and its balance with the atmosphere.”