Manholes are round ….
Because …. Well think about it. What would happen if they were Square?
When you opened a heavy manhole cover , and they are Really heavy…. A square or rectangle or triangle for that matter, could fall in, down the whole!!! There is always a short end that could allow it to slip through the square or rectangular opening. You could left up the top, rotate it then Ooooops …. drop it down the hole.
A circle has the same diameter all the way around so if it can rest on an opening that is slightly smaller than the circumference of the circular lid ….
TaDa! It cannot slip into the hole.