If you were on a battlefield, say, 700 years ago, Damascus steel mattered. The super-strong blades were fabled in their age, said to have sliced through the swords of foes and solid rock. Then we forgot how to make it.
Sixth century metalworkers in India and Sri Lanka were the first to begin crafting the incredibly strong material, shocking Europeans with their weaponry that didn't lose its sharpness and seemed able to cut through anything in its path - as well as gleaming with a dazzling marbled pattern. And underneath it all, visible only now - two and a half centuries later - are amazingly complex carbon nanotubes. Then, abruptly, the weapons started to vanish around 1750. And nobody knows why. More importantly, nobody knows how - the secret behind the nanotech steel's production was lost to history, and hundreds of years of debate and research still hasn't settled the matter.
Some scientists believe the steel's disappearance is due to the loss of raw materials - perhaps Indian ore of special mineral content - that granted the blades their toughness. Other historians argue 'smiths simply had no idea themselves how they were creating the stuff and instead selected from large batches those blades that demonstrated the properties of Damascus steel. Swords weren't exactly decisive factors by the time the Damascus blades began to vanish, but who knows what material breakthroughs we might have reached sooner had we not forgotten the steel's recipe? [TopTenz via Core77]