The Defense Advanced Research Projects Agency (DARPA) is not exactly the most transparent or visually striking stop along the tour, and that's because it's in the process of being relocated. However, DARPA is known as the inventor of Arpanet, the Internet's predecessor.
Based in Arlington, Va., the building itself will be somewhat of a technical marvel. It is expected to meet LEED Gold requirements and the Department of Defense's Minimum Anti-terrorism Standards for Buildings, featuring force protection, 82-foot standoff distance and controlled parking. So though you may feel free to view the building from a distance, don't expect a warm welcome inside -- and please don't take photos.