Much of southeastern part of United States as well as in the Midwest region and the cities along the Mississippi River down to the Mississippi Delta are flat. Well, not 100% flat, but you get the point XD One interesting fact is the city of Birmingham, Alabama is flat in the downtown part but the rest of the area have mountains as the area is in Jones Valley, at the ends of the Appalachian foothills (part of the Appalachian Mountains).
It wouldn't be mountains in Poland, but it surely wouldn't be flat either XD I don't know the word for in in english, but we call them something like "small mountains" or "the thing before mountains." So... basically. It's not flat XD