Geologist Walter Alvarez was working away on some limestone samples in Gubbio, Italy, when he became intrigued by an odd layer of rock. He was looking at the K-T boundary. Underneath it, there are dinosaur fossils. Above it, there are none. And Walter was about to stumble on the reason why. In this final episode of our science series, we pair a rock sample from the K-T boundary with a unique portrait by Carmen Lomas Garza to tell the story of the dinosaur extinction -- how it happened, why it happened, and who figured it out. With Kirk Johnson, director of the Smithsonian's National Museum of Natural History, and Taína Caragol, curator of painting and sculpture and Latino art and history at the National Portrait Gallery. See the portraits we discussed: Walter Alvarez, by Carmen Lomas Garza Luis Alvarez, Lawrence Berkeley National Laboratory