Originally Posted by CheeseFrog

I think that's more of an after-the-fact observation, and it's not entirely true. You can make a perfectly square manhole (or any other shape) in which the cover wouldn't be able to fall through. To make the example easy to imagin, here's an extreme:

A (insert shape here) manhole the size of Texas covering a 1-inch diameter hole. The manhole cover can be any shape you want -- it isn't gonna fall through that hole.