Salar de Uyuni in Bolivia doesn"t ease you in—it erupts into view like a horizon made of light. It"s the world"s largest salt flat, stretching across roughly 4,000 square miles at nearly 3,658 metres above sea level. The landscape dramatically shifts with the seasons: when the rain arrives, the surface floods just enough to become an enormous mirror, so perfectly reflective that sky and ground melt into a single glowing plane.