XP

Manhole covers have been around for a really long time. They were first introduced by the Romans and were made of sandstone. They are also mainly round as a circle can't fall down a slightly smaller hole no matter how you turn it. Due the round shape, they can be transported by rolling and don't to be aligned. Manhole covers usually weigh over 113 kg and are made out of cast iron or concrete.

If you ever see a manhole cover with square pegs, then you can certain that that manhole cover is quite old. The squares were needed to provide a grip for horses and have since lost their main purpose.