A number of possible explanations for why manhole covers are round include :
1. A round manhole cover cannot fall through its circular opening, whereas a square, rectangular, or oval manhole cover could fall in if it were inserted diagonally in the hole.
2. Circular covers don’t need to be rotated or precisely aligned when placing them on the opening.
3. A round manhole cover is easily moved and rolled.
4. Human beings have a roughly circular cross-section.
5. Round tubes are the strongest shape against the compression of the earth around them, so the cover of the tube would naturally be round as well.
6. It’s easier to dig a circular hole.