A23a: World's biggest iceberg on the move after 30 years

The world's biggest iceberg is on the move after more than 30 years being stuck to the ocean floor.

A23a, as it's called, calved from the Antarctic coastline in 1986, but almost immediately grounded in the Weddell Sea to become, essentially, an ice island.

At almost 4,000 sq km (1,500 sq miles) in area, it's more than twice the size of Greater London.

The past year has seen it drifting at speed, and the berg is now about to spill beyond Antarctic waters.