Phys.org:

In Sèvres, a small commune on the outskirts of Paris, lies a gleaming lump of metal the size of a palm. Le Grand K, or Big K as they call the platinum and iridium alloy, sits underground in a high-security vault. It is held under three glass bell jars, and can only be retrieved using three separate keys, each held by different individuals.

Contrary to appearances, tampering and theft isn't the utmost concern for those who guard Big K. Instead, the artifact's custodians have spent recent years worrying that the alloy isn't quite living up to the reputation that it's held for the past century—that it's no longer exactly one kilogram in mass, but micrograms lighter.

Being off by roughly the weight of a grain of sand might seem trivial, but Big K is the International Prototype of the Kilogram. In other words, it's the gold standard by which all other kilograms in the world are measured against. The tiniest discrepancy in Big K's accuracy impacts fields such as medicine, electronics and engineering, sectors where precise measurements are paramount. But a fluctuating kilogram also has rippling effects on other phenomena—such as force, energy and luminous intensity—that use it as the building block for measurements.

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