The annual migration of monarch butterflies is one of science's great mysteries: millions of monarchs know the correct path ...

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.

Monarch butterflies are migrating through San Antonio, laying eggs on milkweed, while other species like the Gulf fritillary ...