Going back as far as the Roman times, in ancient Rome Man has started using stone to protect drain covers. abc
The layman term for drain cover ( Jonite drainage covers ) 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 been invented out from the necessity to prevent 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 current day, the sewage system is still utilised under all cities around the world. Rather than the crude stone manhole covers the Romans used in the ancient days, nowadays we have special steel reinforced stone gratings that provide the thousand year old function of covering the inlets of our sewage system.
Much more than just a cover, check out Jonite's range of drain covers (drain gratings) here: http://www.jonite.us/products/creative-grates