The first residents of Indiana were the Mound Builders, whose burial mounds can still be seen. At the time of the first Europeans, there were relatively few Indians living in the area, although others entered the region after being displaced by white settlers on the east coast. Indiana was first explored by Europeans when La Salle visited during the winter of 1679 to 1680. The first permanent settlement was near the French fort at Vincennes, built in 1702, along the Wabash River. In 1763, the territory was transferred to British control, where it remained until an expedition by Virginians under George Rogers Clark wrested it away in a military campaign during 1778 and 1779. It became part of the Northwest Territories under the Ordinance of 1787. In 1800, the Indiana territory was organized, including all or part of present Indiana, Wisconsin, Minnesota, Illinois, and Michigan. Despite support for slavery by some settlers, the state declared itself against slavery when it adopted the constitution under which it joined the Union in 1816. A period of rampant road and canal building in the 1820s and 1830s, ended in 1840, with the state heavily in debt and many projects uncompleted. Indiana provided many soldiers for the Union side during the Civil War, but only one minor battle took place on its soil. Following the war, Indiana began to develop manufacturing to complement its agriculture. In 1889, Standard Oil built what was then one of the world's largest oil refineries on the shores of Lake Michigan. Production of electric and gasoline-powered automobiles began soon after 1900. The largest steel mill of the United States Steel Corporation was built in Gary, a city itself constructed by the company.