Going back as far as the Roman times, in ancient Rome Man has started using stone to protect drain covers. abc
channel grates drain 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 ever since 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 prefer to call them) have been invented out of the necessity to stop people from falling into underground sewage and to prevent large objects from entering the drains which cause choking which eventually can cause flooding.
Fast forward to the modern day, the sewage system is still employed under all cities all over the world. Rather than the crude stone manhole covers the Romans used in the ancient days, today we have special steel reinforced stone gratings that serve the thousand year old function of covering the inlets of our sewage system.
More than just a cover, check out Jonite's range of drain covers (drain gratings) here: http://www.jonite.us/products/creative-grates