We've long known that there were some issues with France's 'Le Grand K', the international prototype for what a kilogram really is. Made in 1879 from platinum and iridium alloy, it is the perfect standard for what a kilogram weighs. The problem is, it's losing weight.
France holds the world's standard for the kilogram weight. It's a cylinder-shaped object that has dictated all other kilogram weights for 130 years now. As it's made from platinum-iridium though, the US is concerned it's no longer accurate. Whoops?