Missouri lies in the Midwestern United States and shares borders with eight different states. As western expansion progressed, Missouri became known as the “Gateway to the West” because of its great overland routes that led to California and Oregon. This “gateway” concept was symbolized by the building of the Gateway Arch in St. Louis, which is the tallest national monument in the United States. Missouri’s central geographic location makes it a major transportation crossroad with the nation’s largest inland river port on the intersection of the Missouri and Mississippi Rivers and five interstate highways crossing through the state. Missouri’s economy relies primarily on industries such as aerospace and transportation equipment. In addition, machinery, chemicals, printing and publishing, food products, and electrical equipment are major contributors to the economy.