Why Are Manhole Covers Round?

I think there can be several reasons for this 1. the shape accommodates the human form easier 2. no one has to struggle to set it on "just so" in order to fit - thusly saving time (especially important on a busy street) 3. the tunnels or tubes that they set on top of are round - it is easier to dig or drill a round hole. 4. I think this is the main reason - with a circular (not really round, I guess as they are flat) cover the thing won't fall into the hole - it fits the same no matter which way you turn it... with any other shape if it gets cockeyed it could fall in and wedge in to the hole making it a beast to correct and being dangerous in the mean time.

I once read that it's because round ones can't fall into the manhole, whereas square or rectangular ones could if they happened to be turned the wrong way

The are easier to remove then square or rectangular ones and allow workers to get in and out of the holes easier.