(The reason why manhole covers are round is obvious - because manholes are round (which also happens to have additional benefit of not allowing cover to fall into manhole). So the question is why manholes are round?)<p>Manholes are round because of the same reason most wells with brick walls are round - less material is required to support pressure trying to collapse the shaft. I.e. given that manhole needs to reach the same depth, it will be cheaper to have round shaft than, lets say, square, because round shaft walls can be thinner, but still support required pressure.