Perhaps it’s not exactly a question for the ages, but now that I’ve asked, I’m guessing you’re dying to know the answer, right? I mean, if nothing else, it’s exactly the sort of question that will win you a round of Jeopardy, or a game of Trivial Pursuit, or the title of the cleverest person at your next office party, and really, what more motivation do you need in life?
In all seriousness, though, it is an interesting question. Of all the available shapes in the world, why are all manholes circles? Who decided on circles? And why did everyone else go along?
Well, read on for answers, my curious friend.
According to science teacher and author Larry Scheckel, manholes are round because “it is the best shape to resist the compression of the Earth around it.”
Some additional reasons are: circles are easier to manufacture than squares or rectangles and they’re easier to move since circles can be rolled. They’re also easy to slip into place since there are no angles that need to be lined up exactly.
But perhaps what’s most convincing is that a circle cannot fall through a smaller circular opening, no matter how you twist it. Like Scheckel points out, “there is a ‘lip’ around the rim of the hole, holding up the cover, which means that the underlying hole is smaller than the cover. A round manhole cover can’t fall through its circular opening, because no matter how you position it, the cover is wider than the hole. But a square, rectangular, or oval manhole cover could fall in if it was inserted diagonally into the hole.”
And, as you can imagine, that wouldn’t be great for some city worker who’s just trying to put a manhole back and not lose it to the bowels of the underground city.