The layman term for drain cover (grate) is manhole cover. Say "manhole cover" and pictures of spies/heros using 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 would prefer to call them) have already been invented out of the necessity in order to avoid people from falling into underground sewage and to prevent large objects from entering the drains which cause choking which eventually could lead to flooding.
Fast forward to the current day, commercial floor drains is still employed under all cities all over 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 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