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

We have portraits of people in our galleries. But what if you’re a natural science museum? How do you portray a dinosaur?

We talk with Kirk Johnson, Sant Director of the Smithsonian's National Museum of Natural History, about the ways our portrayals of dinosaurs have evolved, from sluggish and lizard-like to warm-blooded, colorful and spry. Then Matthew Carrano, curator of dinosauria, explains how the museum put T. rex into a striking new pose. The trick, he says, is to convey how cool dinosaurs were, without making them seem alien.

We also tie in a couple portraits from the National Portrait Gallery's collection: an image of the first person to describe an American dinosaur, and a photograph of the first person to give them the Hollywood treatment.

See the portraits we discussed:

Joseph Leidy, by Frederick Gutekunst

Steven Spielberg, by Gregory Heisler

The Nation's T. rex, the Smithsonian's National Museum of Natural History

Also recommended:

Visions of Lost Worlds: The Paleoart of Jay Matternes, by Matthew T. Carrano and Kirk R. Johnson

This mini 'Blink' episode celebrates the cherry blossoms that are blooming all over Washington D.C. at the moment by taking a closer look at portraits that feature flowers. Kim visits three paintings in the National Portrait Gallery that use specific blooms to convey coded information about the sitter, including the experiences that shaped them and the roles they took on.

Sarah Weston Seaton with her Children, by Charles Bird King

Barack Obama, by Kehinde Wiley

George Washington Carver, by Betsy Graves Reyneau