Deep in a vault just outside of Paris, a hunk of metal sits vacuum-sealed under three jars. This was arguably the most important piece of metal in the world.
Now, it's just another rock.
Officially known as the International Prototype Kilogram, or "Le Grand K," this piece of metal has been the universal definition of a kilogram since it was forged 1889. The cylinder of platinum-iridium alloy has served as the benchmark for all other weights are compared.
Le Grand K was so important that it was only removed from its enclosure once every 40 years so that its mass can be checked against sister copies housed in other locations across the globe. The process is cumbersome because even hydrocarbons on fingertips or moisture in the air could contaminate the original's surface, which would change the world's definition of the kilo.
That is exactly why scientists have become concerned. Obviously, the mass of a kilogram cannot change even if its benchmark did. And despite their best efforts to keep Le Grand K preserved, it has been losing weight. At its last weigh in 20 years ago, it was found to be 0.05 milligrams lighter than its replicas.
Experts had varying theories, that its replicas were gaining weight since they were handled more often or that Le Grand K was outgassing—air gradually escaping the metal. Granted the difference amounted to basically a grain of sand, but when we're talking about every single scale worldwide, even the smallest change could lead to a massive (pun intended) problem.