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It calved, or broke off, from the Filchner Ice Shelf in Antarctica in 1986 but got stuck on the seafloor and then trapped in an ocean vortex.

Born in a mass breakout of bergs from the Filchner Ice Shelf, in the southern Weddell Sea, A23a was almost immediately stuck in shallow bottom muds to become an "ice island" for more than three decades.

In 1987, a massive iceberg, called A23, broke off from Antarctica’s Filchner Ice Shelf.

The iceberg - equivalent to three times the size of New York City and more than twice the size of Greater London - had been grounded for more than three decades in the Weddell Sea after it split from the Antarctic’s Filchner Ice Shelf in 1986.

The iceberg, known as A23a, split from the Antarctic’s Filchner Ice Shelf in 1986.