Each year, monarch butterflies migrate roughly 2,000 miles from southern Canada to an isolated area in the forests of Mexico. Their great-grandchildren will return to the same spot in what is a multigenerational journey. The award-winning photographer Louie Schwartzberg was there to capture their time in Mexico in his 2013 documentary Wings of Life. What does the sound of a million butterflies flapping their wings sound like? Watch to find out.