Knoxville tried to improve its downtown. The 1982 World's Fair—the most successful of these projects—became one of the most popular world's fairs in U.S. history with 11 million visitors. The fair's energy theme was selected due to Knoxville being the headquarters of the Tennessee Valley Authority and for the city's proximity to the Oak Ridge National Laboratory. The Sunsphere, a steel truss structure topped with a gold-colored glass sphere, was built for the fair and remains one of Knoxville's most prominent buildings, along with the adjacent amphitheater which underwent a renovation that was completed in 2008.