An iceberg is a large piece of freshwater ice that has calved (broken off) from a glacier or ice shelf – and as we all know, only the tip of them is visible above water.
Approximately 90 per cent of an iceberg lurks beneath the water, which makes them deceptively larger than they first appear. This also makes them slightly tricky for scientists hoping to measure just how large they are.
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What’s the largest iceberg in the world?
As icebergs are difficult to measure and constantly changing, the title of the world’s largest iceberg is contested.
One notable contender is iceberg A23a, which was also one of the oldest recorded icebergs. It calved from Antarctica’s Filchner Ice Shelf in 1986 and spent more than 30 years grounded in the Weddell Sea. It then began its journey further north in 2020, slowly fragmenting as it went.
At its peak, iceberg A23a measured 4,000 square kilometres (approximately 1,540 square miles) – around twice the size of Greater London – and weighed nearly a trillion tonnes. It lost its world-record status at the end of 2025 to an iceberg known as D15A.
Another contender for the largest iceberg ever was estimated to measure more than 31,000 square kilometres (12,000 square miles) – that’s larger than Belgium. It was sighted in 1956 off Scott Island, in the Southern Ocean, by the USS Glacier. However, its size was only estimated as the crew onboard didn’t have the use of satellite photography that is used today – so we can never know for sure how large it really was.
The third contender is iceberg B15, which is the largest iceberg to be assessed using satellite imagery. According to NASA, it had a surface area of 11,000 square kilometres (4,250 square miles) and measured approximately 300 x 40 kilometres (186 x 25 miles).
However, as icebergs are constantly forming, moving and melting, the record-holder is also frequently changing. Scientists study the movement of icebergs as they can significantly impact nearby ecosystems – for example, by blocking off food sources for specific species.
Icebergs also influence ocean cycles of carbon and nutrients in polar regions and on a global scale. They also indicate how polar regions might respond as the climate changes.
Top image: iceberg A23a floating in the Southern Ocean in 2024. Credit: Samuel J Coe/Getty Images
