A manhole cover is a removable plate forming the lid over the opening of a manhole, to prevent anyone or anything from falling in, and to keep out unauthorized persons and material. Manhole covers date back at least to the era of ancient Rome, which had sewer grates made from stone. Manhole covers are often made out of cast iron, concrete or a combination of the two. This makes them inexpensive, strong, and heavy, usually weighing more than 50 kilograms. Manhole covers may also be constructed from glass-reinforced plastic or other composite material. Common metallic manhole covers corrode easily and they get robbed for recycling. Therefore, polymer manholes are developing worldwide due to their low production cost, high mechanical and hydraulic resistance and also having chemical and corrosion resistance.