Tens of millions of monarch butterflies set out every August on a six-week 3,000-mile (5,000-km) journey from Canada and the northern U.S. to a destination they have never seen in Mexico. There they gather for the winter in El Rosario Sanctuary, covering trees, sometimes flying up from winter naps in a softly-whispering blizzard of black and orange insects. In spring they will reawaken, mate, and their descendants will return to the same places their ancestors came from—exactly how, no one knows—but the next generation will make the same miraculous-seeming journey the following year.