During World War II, the U.S. needed greater aluminum supplies to build airplanes. Aluminum plants required large amounts of electricity. To provide the power, TVA engaged in one of the largest hydropower construction programs ever undertaken in the U.S. By early 1942, when the effort reached its peak, 12 hydroelectric plants and one steam plant were under construction at the same time, and design and construction employment reached a total of 28,000. In its first eleven years, TVA constructed a total of 16 hydroelectric dams.