For thousands of years, millions of monarch butterflies have migrated from the northern U.S. and Canada to Mexico for the winter. But exactly how they know where to go is still a mystery.

Monarchs traveled to a remote part of Mexico, a journey they've made for thousands of years, and wintered there. Now tens of millions of the butterflies are on an epic aerial journey back north.

Up to a billion monarch butterflies return annually, from breeding areas as far away as Canada, to land in close-packed clusters within 14 overwintering colonies in the oyamel fir forests of central ...