Why are manhole covers round ?

Below are the reasons :

A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it were inserted diagonally in the hole. (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture. The existence of a "lip" holding up the lid means that the underlying hole is smaller than the cover, so that other shapes might suffice.)

Round tubes are the strongest and most material-efficient shape against the compression of the earth around them, and so it is natural that the cover of a round tube assume a circular shape.

The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using a lathe.

Circular covers do not need to be rotated to align with the manhole.

A round manhole cover can be more easily moved by being rolled.

A round manhole cover requires the least amount of metal to cover an opening wide enough for a person to get through

A round manhole cover can be easily locked in place with a quarter turn (as is done in countries like France). They are then hard to open unless you are authorised and have a special tool. Also then they do not have to be made so heavy - traffic passing over them cannot lift them up by suction.

Source : Wikipedia