Here's my take (an unemployed ironworker) on the manhole cover question: 1. Safety. A manhole cover sits on a flange that keeps it approximately level with the surrounding pavement. In other words, the outer diameter of the cover is greater than the inner diameter of the mounting flange. Therefore, it does not drop through the hole (duh). However, a cover that was square in shape could possibly drop through. That is to say that if the manhole cover was placed at an angle, the measure of its side would be less than the hypotenuse of the hole, and it would fall through. For one, it is not that easy to pick a cover up over head from underneath anyway, but imagine trying to pick a fallen cover out of the sewer and carry it back up that little ladder. Not easy. 2. Labor. A manhole access point does not just exist in nature, it has to be created. A drill or auger can be used to drill down into the sewer system, but you cannot drill a square hole. In other words, an auger makes round holes; that is just the way it works. Why go to the extra effort and man hours to make a rectangular hole, when a round hole will work just fine? 3. Material. Steel is not free. Assuming equal thickness, a round hole with diameter X will require an area of steel equal to 3.14*(1/2X)^2 = .785*X^2 while a square with side length X would require 1*X^2 in steel. So there is a material savings of over 20% while still keeping the same basic dimensions as far as usable accessibility goes. 4. Production. A manhole casting that is round can be mounted on a lathe to theoretically finish its edge in one operation. However, a square manhole casting would need to be mounted in a special setup on a mill. This would theoretically require four machining operations. So it is just easier to produce that circular shape. 5. Installation. You cannot roll a square shape. You don't always want to have to pick up a solid steel cover, so it helps that you can pretty much just roll it into place. Who wants to carry a big hunk of steel towards the end of a long day? It's kind of funny that a question like this can get you college grads a good paying job when I can't even manage to find a job as an ironworker right now. This economy is maddening; those at the top prosper while the rest of us get left behind.