100. The Pentagon, the world’s largest office building and headquarters of the United States Department of Defense, was not completed until January 1943, when Groves was fully engaged in the Manhattan Project. He had worked on the project for a year. The Pentagon was originally designed as a Veteran’s Administration hospital, and was intended to put all the disabled World War I veterans needing medical care at one location. This partly explains the odd structure of the building. It was never put to its intended use.