The largest plain in the world is the Amazon Basin, which is located in South America. It covers an area of approximately 7,000,000 square kilometers (2,700,000 square miles), making it larger than the entire continent of Australia.

While the Amazon Basin is considered a plain, it's important to note that it's not a flat, featureless expanse. It's characterized by a vast network of rivers, swamps, and forests, with elevations ranging from sea level to a few hundred meters.