Illinois is a state in the Midwestern United States. It is the 5th most populous and 25th most extensive state, and is often noted as a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal, timber, and petroleum in the south, Illinois has a diverse economic base and is a major
transportation hub. The Port of Chicago connects the state to other global ports from the Great Lakes, via the Saint Lawrence Seaway, to the Atlantic Ocean; as well as the Great Lakes to the Mississippi River, via the Illinois River.
Illinois' eastern border with Indiana consists of a north-south line at
87° 31′ 30″ west longitude, from Lake Michigan to the Wabash River above Post Vincennes. The Wabash River continues as the eastern/southeastern border with Indiana until the Wabash enters the Ohio River. This marks the beginning of Illinois' southern border with Kentucky, which runs along the northern shoreline of the Ohio River. Most of the western border with Missouri and Iowa is the Mississippi River; Kaskaskia is an exclave of Illinois, lying west of the Mississippi and reachable only from Missouri. Its northern border with Wisconsin is fixed at
42° 30' north latitude. The northeastern border of Illinois actually lies within Lake Michigan, within which Illinois shares a water boundary with the state of Michigan.
Though Illinois lies entirely in the Interior Plains, it does have some minor variation in its elevation. In extreme northwestern Illinois, the Driftless Area, a region of unglaciated and therefore higher and more rugged topography, occupies a small part of the state. Charles Mound, located in this region, has the state's highest elevation above sea level at 1,235 feet (376 m) 1,235 feet (376 m). The floodplain on the Mississippi River from Alton to the Kaskaskia River is known as the American Bottom.
Illinois has three major geographical divisions. Northern Illinois is dominated by Chicagoland, which is the city of Chicago and its suburbs, and the adjoining exurban area into which the metropolis is expanding. As defined by the federal government, the Chicago metro area includes several counties in Illinois, Indiana, and Wisconsin, and has a population of over 9.8 million people. Chicago itself is a cosmopolitan city, densely populated, industrialized, and the transportation hub of the nation, and settled by a wide variety of ethnic groups. The city of Rockford, Illinois' third largest city and center of the state's fourth largest metropolitan area, sits along Interstates 39 and 90 some 75 miles (121 km) northwest of Chicago.
The midsection of Illinois is a second major division, called Central Illinois. It is an area of mostly prairie and known as the Heart of Illinois. It is characterized by small towns and medium-small cities. The western section (west of the Illinois River) was originally part of the Military Tract of 1812 and forms the conspicuous western bulge of the state. Agriculture, particularly corn and soybeans, as well as educational institutions and manufacturing centers, figure prominently in Central Illinois. Cities include Peoria, Springfield, the state capital; Quincy; Decatur; Bloomington-Normal; and Champaign-Urbana.