Princeton University manhole cover, Princeton, NJ, USA

Pick holes in manhole cover, Palo Alto, CA, USA

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A maintenance cover or manhole cover is a removable plate forming the lid over the opening of a maintenance hole, to prevent someone from falling in and to keep unauthorized persons out. more...

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Manhole covers usually weigh more than 100 pounds (roughly 50 kg), partly because the weight keeps them in place when traffic passes over them, and partly because they are often made out of cast iron, sometimes with infills of concrete. This makes them inexpensive and strong but heavy. They usually feature "pick holes", in which a hook handle is inserted to lift them. Specialized manhole hooks are manufactured for this purpose.

India has become the world leader in manufacturing manhole covers, driving many manufacturers in other countries out of business. Its extremely low labor costs have overcome even the cost of shipping such heavy objects.

Although the covers are too large to be collectible, their ubiquity and the many patterns and descriptions printed on them has led some people to collect pictures of covers from around the world.

In some areas the term "manhole" is now regarded as a sexist term, especially now that women are increasingly working in the utility sector. The term "maintenance cover" is used by some while others prefer "maintenance hatch" and "maintenance hatch cover" to preserve the acronyms "MH" and "MHC" on diagrams.

Despite their weight and cumbersome nature, manhole covers are sometimes stolen, usually for resale as scrap, particularly when metal prices rise.

Why are they round?

The question of why manhole covers are typically round, at least in the U.S., was made notorious by Microsoft, and Tony Martinez when they began asking it as a job-interview question. Originally meant as a psychological assessment of how one approaches a question with more than one "right" answer, the problem exerts a strange fascination on people, who seek to find alternate explanations, from the pragmatic and satirical to the philosophical. One practical answer being, "Manhole covers are round because manholes are round."

Reasons for the shape include:

A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it were inserted diagonally in the aperture (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.); Round tubes are the strongest and most material-efficient shape against the compression of the earth around them, and so it is natural that the cover of a round tube assume a circular shape.; Cylindrical holes are easier to dig.; The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic.; Round castings are much easier to machine using horizontal boring mills.; Circular covers do not need to be aligned to put them back.; A round manhole cover can be more easily moved by being rolled.; If a automobile rolls over a dislodged manhole of another shape, the sharp corners could puncture the automobile's tire, whereas a circle doesn't have corners.; Tradition; Aesthetics;