On the outskirts of Paris, a very important chunk of metal is carefully stored away and monitored. What’s so important about this 125-year old metal chunk?

Well, its mass is the very definition of a kilogram that all the scales in the world are ultimately calibrated against. Scientists have recently discovered a disturbing problem with this important piece of metal. It appears to be losing mass. The six originally identical copies stored with it as well as other copies throughout the world stored in similar conditions are also experiencing changes to their mass. With the original standard measurement of what a kilogram fluctuating, scientists are racing to redefine the kilogram with a better replacement, a close to perfect sphere made of silicon. Derek Muller of Veritasium Science Education explains in the following video the history behind this very important piece of metal and the global efforts to find a replacement for it.

November 2nd, 2015: Here’s an update on this story. A solution has been adopted and will be in place by 2018.