Quote Originally Posted by Comintern
I thought they were round because circles provide the most efficient ratio of surface area to width.
If you wanted to minimize the surface area of the manhole, then you'd want to choose a shape that represents the profile of people as viewed from above (or below) and choose the minimum size so that someone could just barely squeeze through. This shape isn't round .... well, OK, for the people who typically must work in the manholes, it that shape might be round.