I never thought about why manhole covers were round before reading this detailed history of manhole covers in New York City. That they are round “so they won’t fall into the manhole … if you insert [them] at an angle or vertically” makes sense. Maybe I never thought about it because I have never been tasked with installing a manhole cover.
Before we close out, have you figured out yet why are manhole covers round? My contact at the DEP says, “The principal reason that manhole covers are round is so they won’t fall into the manhole.” If a manhole is square, rectangular or even oval, it can fall into the manhole if you insert it at an angle or vertically. Yikes!