The world’s largest iceberg is on the move again after being stuck for 30 years in the Weddell Sea. A23a is 3900 square kilometres in area, making it more than twice the size of Greater London and more than four times the size of New York City. It is 400 metres thick and it once housed a Soviet research station. Satellite images from the European Space Agency’s Sentinel-1 satellite, processed by the British Antarctic Survey, are tracking it as it crosses the Weddell Sea and passes the northern tip of the Antarctic Peninsula.