Here's why it's considered the flattest:

* Vast and Uniform: Salar de Uyuni spans over 4,000 square miles (10,582 square kilometers), making it the largest salt flat on Earth.

* Minimal Elevation Change: The elevation difference across the entire surface is less than one meter.

* Perfectly Smooth: The salt crust is incredibly smooth and uniform, with only a few minor undulations.

This flatness is what makes Salar de Uyuni a unique and visually stunning location.