Manhole cover
A manhole cover is a removable plate forming the lid over the opening of a manhole, to prevent someone from falling in and to keep unauthorized persons out.
Manhole covers usually weigh more than one hundred pounds (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 up.
The question of why manhole covers are typically round, at least in the US, was made notorious by Microsoft, who began asking it as an interview question. Originally meant as a psychological assessment of how one approaches a question with more than one "right" answer, the problem seems to have exerted a strange fascination on people to find alternate explanations, from the pragmatic and satirical to the philosophical.
In reality, manhole covers are round because manholes are round. Round tubes are the strongest and most material-efficient shape against the compression of the earth around them. A circle is the simplest shape whereby the lid cannot fall into the hole. (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.) Circular covers can be moved around by rolling. And they need not be aligned to put them back.
However, other manhole shapes can be found outside the U.S. - usually squares or rectangles, but hexagons, triangles and other shapes exist. So the reasons for the shape cannot be entirely as suggested by the popular answers to 'Why are they round?'. Nashua, New Hampshire, U.S.A., appears to be unique in the USA with triangular manhole covers that point in the direction of the underlying liquid flow. The city is phasing out the triangles because they are not large enough to meet modern safety standards.
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.
Political correctness
It has been suggested that the gender-specific term "manhole cover" is not politically correct, but the gender-neutral version "person-hole cover" does not seem to have caught on. Alternative suggestions include "inspection cover" and "access hatch".
The true first man-made object in space?
According to urban legend, a manhole cover was accidentally launched from its shaft during an underground nuclear test in the 1950s, at great enough speed to leave the solar system. The myth is based on a real incident in 1957 during the Operation Plumbbob nuclear tests, where a heavy steel plate cap was blasted off the test shaft at tremendous velocity, never to be seen again, although it is doubtful that it left the atmosphere.