Why are manhole covers often round?
The Real Answer
There is not one universal reason, but round manhole covers solve several practical problems at once. A circular cover that rests on a smaller circular opening cannot slip through simply because it is rotated, while some non-round covers can fit through their openings at certain tilted orientations.
Round covers also do not have corners that must be aligned when the cover is put back, and a heavy round cover can be rolled on its edge instead of being carried the whole distance. Circular openings and covers can also be a straightforward, economical fit for round access shafts.
Those advantages explain why round covers are so common, but the shape is not a law of nature or a universal requirement. Square, rectangular, and other utility covers also exist when a different design fits the job.
You walk straight across a removable metal lid over an underground access shaft without breaking stride, then sidestep a loose sidewalk brick two feet later. You never check what is holding the cover up; you put one foot in the center and keep walking.
Why People Ask This
People ask because the repeated circular shape looks too deliberate to be decorative, and the obvious alternative — a square lid — seems as though it should work. The answer is interesting because the circle is not about one secret rule; it combines geometry, handling, alignment, and ordinary infrastructure practicality.