This is the first of a two-part episode on "infrasound," or sound that lies below the threshold of human hearing. Infrasound can warn of impending danger, and it can explain seemingly inexplicable animal behavior.

In part 1, we go to Hawaii and Chile with an volcanologist who records the voices of volcanos, looking for subtle shifts in their infrasound that could signal a pending eruption. Then we go up the coast to Guatemala, to detect mudslides triggered by a volcano—before they happen, giving people time to get out of the way.

Guest: Jeffrey Johnson, Associate Professor of Geoscience at Boise State University