Manhole covers are usually circular in shape, made of iron and found on top of manholes. Manholes are entry points, located on the ground, which usually provide access into the underground sewerage or utility areas of a city. Curiously enough, they tend to be located in the middle of roads, and usually have fast-moving traffic passing over them. It is not known whether or not this was or is a basic form of manhole security. If it is, then it isn't a very good one, as at night most roads are deserted.
From as early as 3500BC, slabs of stone or pieces of wood were used to cover the ditches that carried waste. Several civilisations developed this further - for example, the Romans had a complex system of waterworks - but sewerage system designs that are implemented today were not developed until the 18th Century. As time progressed, tunnels or tubes were used to dispose of waste in order to stop the spread of infectious disease. As the population grew in locations with sewers, there became a need to tie these systems together so that the waste could be removed more efficiently. These sewage lines had a hole dug between the last points in each line in order to connect the two systems together. A cover was then placed over the junction point. This junction served as a plumbing point in case the sewer became clogged with debris and backed up. It was at some point during the 19th Century that manhole covers started being made of iron.
The word manhole was used in the past to describe particular areas on decks of sailing ships. These manholes provided access to goods and stores in the hold and measured approximately a metre long by a metre wide. An access hatch was constructed and placed over the top of the manhole to ensure that nobody would gain a practical experience of the laws of gravity while standing directly over the manhole.
Today these openings are known as companionways or hatches. Another odd titbit is that this space is scarcely large enough for a single man to fit through.
Why Manhole Covers are Round
The fact that most manholes in the USA are round has given rise to much speculation, and several popular explanations have arisen1:
You can roll them, and thus transport them by hand, more easily than triangles or squares.
A circular tube will displace ground pressure more effectively than any other shape.
The shape makes them much easier to line up than triangular and square covers.
Most other shapes could relatively easily be turned on their side and fall through the hole.
Of course, not all manhole covers are round. Rectangular and triangular covers are also known. Whatever the shape of the manhole, the manhole cover bottom is recessed about 2.5cm so it doesn't fall through, since the inner diameter of the manhole is smaller than that of the manhole cover.
Interesting Facts about Manhole Covers
Nashua, New Hampshire, in the US has triangular manhole covers that point in the direction of flow but that design is currently being phased out in favour of round covers.
Triangular manhole covers do not rattle like round manhole covers do, but they tend to stick due to expansion from heat.
Manhole covers are made of iron or concrete for larger systems where weight is a concern.
Most iron manhole covers have an artistic or aesthetic print forged on them. Some are smooth, while others have a pattern to assist traction of vehicles passing over them.
Despite generally weighing more than 50kg, manhole cover pictures are sometimes collected as works of art.
The forged print of a manhole cover almost always contains an identifying mark for the company that made it.
In Rome, manhole covers carry the legend 'SPQR', an abbreviation for 'Senatus Populusque Romanus', 'The Senate and People of Rome.'
The size of a manhole cover varies but measures approximately 60.96cm radius in the USA.
Unlike in movies, where a person would fall through a manhole as a joke and survive with minimal injuries, if you were to fall into a manhole unsuspectingly you would be seriously injured or could possibly even be killed from the fall.