The migration on Monarchs is thought to 10,000 years old coinciding with the end of the last ice age. And yet, their winter resting grounds weren’t known till 1975.  

The high altitude forests that provide winter habitat for hundreds of millions of monarch butterflies are on the borders of the states of Mexico and Michoacan. 

In 1986, the Mexican government created the Monarch Butterfly Biosphere Reserve (Mariposa Monarca Biosphere Reserve) to protect 62-square miles of forests within four separate monarch sanctuaries. 

The Biosphere Reserve was expanded to include 217 square miles in 2000.