Man-hole covers (inspection covers) are made from heavy, thick cast-iron. Gates can be made from wood or metal.
Because they are meant for a man to go through them
There are approximately 100,000 manholes in Manhattan.
Iron is the main ingredient, but it usually contains carbon too.
Manholes are human access points for when the sewer system needs maintenance or repair.
Manhattan has approximately 800,000 manholes. These manholes are part of the extensive network of utilities and infrastructure that includes sewage, water, and electrical systems. The high number reflects the dense urban environment and the need for maintenance and access to underground services.
If they were rectangular they could fall down the manhole.
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Bill GATES USED TO BE THE RICHEST MAN IN THE WORLD
Outside diameter (OD) of adjacent manholes (including the bottom slabs) should have minimum of five (5) feet horizontal clearance.
"Life is not fair; get used to it." , Bill Gates
light gates are used to brighten up the entrance when it is dark.
The greyhound gates at the Port Authority are used for boarding and departing Greyhound buses.