Southern Illinois is a region of the U.S. state of Illinois comprising the southern third of the state, principally south of Interstate 70. Part of downstate Illinois, it is bordered by the two most voluminous rivers in the United States: the Mississippi below its connection with the Missouri River to the west and the Ohio River to the east and south, with the tributary Wabash River, extending the southeastern border. Some areas of Southern Illinois are known historically as Little Egypt. Although part of the Midwest, certain areas of Southern Illinois more closely align culturally with neighboring parts of the Upland South (i.e. Kentucky, Tennessee, Southern Indiana, and Missouri).
Southern Illinois' most populated city is Belleville at 44,478. Other principal cities include Alton, Centralia, Collinsville, Edwardsville, Glen Carbon, Godfrey, Granite City, O'Fallon, Harrisburg, Herrin, West Frankfort, Mt. Vernon, Marion, and Carbondale, where the main campus of Southern Illinois University is located. Residents may also commute to St. Louis and Cape Girardeau, Missouri; Evansville, Indiana; and Paducah, Kentucky. The region is home to Scott Air Force Base, a major military installation.