Container ships are the monsters of the seas - the very biggest are almost half a kilometre long and piled high with up to 20,000 huge boxes. At any one time, there are tens of thousands of these floating cities on the move, many unable even to dock at local ports. It’s our relentless demand for more and cheaper stuff that drives the industry. We meet the British salvage man who’s making millions from the boxes that get left behind, lost or abandoned - yet another example of how invisible the world of shipping is, even though the whole planet depends on it. Image: Jake Slinn, founder of JS Global, at Felixstowe docks