"Radio astronomy can unmask structures in the universe that are otherwise invisible. And the discoveries are spectacular. One of the most ambitious projects has been the mapping of the structures of the Milky Way, an immense task that has taken decades. Using small radio telescopes in the US and Chile, Thomas Dame's team has made a series of great discoveries, including the identification of two previously unknown spiral arms in the Milky Way. In this lecture, Dr. Dame explains how radio telescopes reveal the invisible, why mapping the structure of our own galaxy is so difficult, and how the famous discoveries of this project were made. Thomas Dame, PhD, Director, Radio Telescope Data Center, Harvard-Smithsonian Center for Astrophysics, and Lecturer on Astronomy at Harvard University"