Do you people even watch the movies before whining about them? They were running out of supplies, many people had already gone crazy, most learned people were dead aside from the crazy blimp inventor, and all the Atolls were not only running out of supplies but were almost all destroyed by the smokers by then. They don't have supplies to dredge the bottom. They don't have educated people that would even think of such a thing. And they have far more urgent problems they are worrying about than just getting dirt.