Out of sight, over the horizon, possibly out of mind as well - but still pumping out pollutants! There's one massive transport network that needs cleaning up, but which we might have forgotten - shipping. There are 110,700 ships of over 100 gross tonnes ploughing through the world's oceans. Although mile for mile and ton for ton ocean shipping is be far the cheapest way to transport goods, a great deal of energy is needed to push them through the water. Though fitted with enormous, relatively efficient, diesel engines, the "bunker fuel" that most large ships use for fuel is basically the unrefinable junk that the petroleum discards - something like the tar that is used on roads. Not only is it black, sticky and unheathily dangerous, but it is rich in pollutants such as sulphur and produces clouds of black poisonous smoke full of dangerous micro-particles.