We've known for a few hundred years now, the most knowledgeable of us knew thousands of years ago, the Earth isn't exactly flat. In fact, it's quite "un-flat" at any scale you pick. If you look around, you'll probably notice that the terrain is quite bumpy even across small distances. And, of course, the entire planet is round, more or less.There are a few incredibly flat places on Earth though, like most dried lakebeds. Salt flats aren't named like that for nothing and the flattest of the salt flats is in Bolivia, Salar de Uyuni.
It's a very arid region, yet there is a constant water supply from the Lauca River nearby. These conditions combine to make the perfect salt flat. Water feeds into the Coipasa Lake, but the water level drops significantly during drought spells.
As the water evaporates, salt is left behind deposited one fine layer at a time. It's this process that creates the incredibly flat surface. So flat in fact that the region is used for satellite calibration.
But as the water starts to build up again over the flat surface, the world's largest mirror is created as the very shallow water is almost perfectly still.