The monarch butterfly completes one of the longest and most mysterious migrations on Earth, and it happens almost entirely across the United States. Every year, millions of monarchs travel from Canada and the northern U.S. all the way to central Mexico, covering up to 3,000 miles — yet not a single butterfly completes the full round trip. Here’s the mind-blowing part: The monarchs that return north are great-grandchildren of the ones that left. No GPS. No leaders. No parents guiding them. Yet th