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A manhole, or maintenance hole, is a tube which connects underground utilities to the surface. The tube is large enough to accommodate the body of a utility worker, and is covered with a top that is flush to the ground, and called a manhole cover. Manholes can be found connected to water and sewer systems, along with electrical and fiberoptic cabling. In all cases, manholes are located at primary junctions, and sometimes at intervals along the utility line, to allow workers easy access to potential problems.
In some instances, a manhole leads into an underground utility vault, an enlarged room where switching equipment can be found. An underground vault may be accessed on a routine basis, to hook up new customers, perform safety checks, and ensure that other key maintenance is performed. Typically, the underground vault will be relatively small, able to accommodate one utility worker at a time.
In very large underground systems, manholes connect to a series of underground pipes, some of which are large enough to walk and move around in. Workers can check pipes for obstructions, make sure that the pipes are in good condition, and identify potential problem areas which need to be serviced. Navigating in this environment requires protective clothing and a good sense of direction, as some sewers have pipes which were built centuries ago, and they can be both dirty and confusing to move around in.
The manhole cover is the visible part of the manhole which most people interact with. A manhole cover is typically round, to prevent it from falling into the manhole and also to make it easier to put back when the work is finished, as no corners need to be matched. In addition, the rounded edges are less likely to catch the tires of cars and bicycles. Typically, when a manhole cover is removed to access the utilities beneath, a protective barrier is put up to prevent people from falling in, and so that nothing is inadvertently dropped onto the utility worker underground.
Many cities have manhole covers made by a variety of utility companies, which can provide an interesting pictorial history of active utilities in the city, along with the progress of industrial design. The cover is usually made to appear somewhat ornamental, and also features the name of the utility which installs it. Covers may be painted or have ornate patterns cast into them, and they usually have small holes for a utility worker to insert a pick, a device designed for lifting manhole covers.
@pleonasm - One of the nice things about manhole covers is that they often have lovely pictures on them that relate to the city they are in. I'm not sure if it's the same company that makes all of them, but it's often almost like art and I think it's lovely that they make an effort to make something that's basically the lowest part of the city into something that people can be proud of.
I also like it when they make sure to label which manholes aren't safe as dumping areas. People don't realize that often the drainage in big cities goes straight into the sea and dumping chemicals into there is a really bad idea.
One of the things that Google used to do when they were trying to hire the best people they could find was to ask strange questions in their job interviews. One of the more famous questions was "why are man covers always a circle?" If you have a look online, you can actually find a lot of really good answers to this question. Like, for example, the general shape of a person is round, that it makes it easier to ensure the cover won't fall into the sewers, that it can be rolled and therefore is easier to move and so forth.
Although apparently the main reason is that the original company that made the covers decided to make them in circles and everyone just copied them because once the infrastructure was in place there wasn't any other choice.
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