It's impossible not to notice them. You've likely also noticed that everywhere in the world, manhole covers are round. This is also a question which you may be asked in an interview and was famously asked to job applicants applying at Google...this happens so much it's included in (The Complete Idiot's Guide to Job Interviews.) If you're asked this question, the interviewer isn't so much concerned with the correct answer, rather, they're wondering how you'll go about coming to an answer. The question is an easy way to gauge your problem solving skills and ability to think on your feet.
According to Larry Scheckel in his book: Ask A Science Teacher: 250 Answers to Questions You've Always Had About How Everyday Stuff Really Works.
Manholes are round because “it is the best shape to resist the compression of Earth around it.” Round shapes are easier to manufacture than square or rectangular shapes, and because manhole covers are heavy, being round makes them easier to move from place to place (just roll them!). As an added benefit, workers don’t need to line up the covers with any angles, making round covers easy to slip into place.
But perhaps the biggest reason that manhole covers are round is that round covers can’t fall through a circular opening. “For all manholes, 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,” Scheckel writes. “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,” which would be bad news for unobservant pedestrians and drivers alike.
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