Why are
manholes round you ask? Well, there's a pretty simple
reason for it. A round manhole (and specifically the
manhole cover) is the easiest shape to make that won't fall into its own hole.
You can make a
square manhole cover that won't fall down the manhole, but to do this you need the cover to be enough larger than the opening that the
side of the square is larger than the
diagonal of the opening. This is the advantage of the round cover, the closest it has to a
diagonal is its radius.
There are some other reasons for having round manhole covers, such as the fact that round pipe is much easier to
mass produce, and you have a more even
distribution of forces in the wall of the manhole, but this is the biggest reason.
Update: mr100percent has informed me that another advantage of this is that you can roll the covers. I hadn't thought of that, but it makes plenty of sense.