The Great Plains is a vast expanse of prairie and flatlands extending across the middle section of the United States. The area is characterized by flat grasslands or rolling hills with relatively few trees. The area has been historically used for farming, ranching, and manufacturing. Some areas of the Great Plains are completely devoid of people, and others have just four or five per thousand square miles. Much of the Great Plains is known as "Tornado Alley" because most of the nation's tornadoes occur here. The Great Plains area is the home of the American Bison. Millions once roamed here before westward expansion occurred in the 1800's.