Constant Wonder presents the second of a two-part episode on "infrasound," or sound that lies below the threshold of human hearing. Infrasound can explain seemingly inexplicable animal behavior. In part 2, we meet Hawaiian insects and African elephants that use the same technique to hear sounds not audible to the human ear. They both can hear with their feet! And while we don't hear with our feet, research on these animals may help improve hearing aids in the future—because we do actually hear with our bones. Caitlin O'Connell-Rodwell, instructor at Harvard Medical School, elephant expert, and co-founder and CEO of Utopia Scientific