Salar de Uyuni in Bolivia is the world's largest salt flat, which transforms into 'the world's largest mirror' (used for calibrating the altimeters of Earth observation satellites) during the rainy season, when a thin layer of water covers the surface. This creates a breathtaking natural phenomenon where the sky and clouds are perfectly reflected, offering surreal and stunning views.
Edit - images from https://www.thelegacyuntold.com/destinations/the-worlds-largest-natural-mirror/