Why are manhole covers round instead of square?

The main reason that manhole covers are round is because a round cover cannot slip through the hole, whereas a square cover could slip through the hole diagonally. Round covers are also easier to manufacture and maneuver.

The round shape also means that heavy manhole covers can be rolled to move them. It is also easier to dig a circular hole. Another reason that manhole covers are round is that the round shape is same shape all the way around, which makes it easier to put back on. Round tubes are also the strongest shape against the Earth's compression, so the cover of the tube should also be round.