Atlas Obscura and The Monarch Butterfly

"Each year, as winter gives way to spring, tens of millions of monarch butterflies leave the safety of their wintering grounds in central Mexico and migrate north. But the history behind how the global scientific community finally came to understand the unique role that this part of Mexico plays in the lifecycle of monarchs is just as fascinating as the butterflies themselves."

https://www.atlasobscura.com/videos/gaze-upon-a-million-monarch-butterflies-in-mexico