In the mid-20th century, whale populations were dwindling. More than 50,000 whales were killed each year by commercial whalers.
But then in the 1960s, a song — or rather, many songs — sparked a movement.
It started with some underwater equipment that, for the first time, captured the sound of humpback whales.
At his home in Vermont, biologist Roger Payne plays the audio that was discovered back then. He points out themes in the whales' song, and how they evolve over time.