What flat lands cover most of Nebraska and are nearly surrounded by mountains?

The only flatlands in Nebraska are along Interstate 80 which runs parallel to the plat river. The flood plain is flat and is about 40 to 50 miles wide in some areas. This runs East and West along the entire state. To the west of this are the sandhills (now I am not sure if whomever is calling these mountains, but they are hills). And to the east of this are smaller, rolling hills.