Going back as far as the Roman times, in ancient Rome Man has started using stone to cover drain covers. abc
The layman term for drain cover (grate) is manhole cover. Say "manhole cover" and images of spies/heros making use of the underground sewage system popping out of manhole cover in Hollywood films appear.
Historically from the time that Man invented the sewage system since the Roman Times to drain water and urban waste away for sanitation purposes, manhole covers (drain covers or grates as we would prefer to call them) have already been invented out from the basic need in order to avoid people from falling into underground sewage as well as to prevent large objects from entering the drains which cause choking which eventually may cause flooding.
Fast forward to the present day, the sewage system is still being used under all cities worldwide. As opposed to the crude stone manhole covers the Romans used in pool grate , now we have special steel reinforced stone gratings that provide the thousand year old function of covering the inlets of our sewage system.
A lot more than a cover, check out Jonite's range of drain covers (drain gratings) here: http://www.jonite.us/products/creative-grates