Illinois is a state in the midwestern region of the United States. It is a mostly flat plains state, with the majority being farmland except for the river valleys and the south, which are hilly and forested. Often noted as a microcosm of the entire country, Illinois has long had a reputation as a bellwether both in social and cultural terms, and politics. 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. For decades, O'Hare International Airport has been ranked as one of the world's busiest airports.