Does everyone remember that day in science class when they told you the Earth was round, and you were like, "Yeah, no duh." And then a couple of years later they told you the Earth was also incredibly smooth, and you kinda just went with it. And then they said if you held an orange and a billiard ball in your hand, and then you could also hold the Earth, that it would feel more like the billiard ball than it would the orange? And that's when you got real confused. What about all the super tall mountains and super deep valleys and the trenches and the oceans? It made sense eventually though that these features were rather small compared to the whole mass of the planet. So now the Earth is one of the smoothest things you can think of. But there actually is one thing on Earth that is the most roundest, smoothest thing to exist on our planet: a ball of silicon. This sphere is officially the roundest thing humans have created; if it were Earth, the distance between its highest and lowest point would be 14 meters.
The reason it was created is pretty cool too. The sphere is an attempt to create a substitute fir the current definition of the kilogram, as this measurement is currently dependent on a physical object called "The Big K" that is locked away in some basement in France. The problem is that there is no way to keep this object from being warped over time, and the definition of a kilogram therefore changed. The creators of the silicon sphere hope to use the amount of atoms in that sphere as a new constant that would replace the kilogram.