The present Monarch Butterfly Biosphere Reserve was established in 2000 and consists of more than 138,000 acres of high mountain area along the border between the states of Michoacan and Mexico. More than 150 million Monarch butterflies from North America winter in this area.

El Rosario is the largest of the four sanctuaries and probably the most accessible. Vehicles can ascend as high as 10,000' after which visitors must trek another 800 feet upwards along a one-mile long trail to the largest congregations of butterfly. The well-marked and maintained trails are also excellent for a variety of high altitude birds.

At the end of the trail unbelievably massive clusters of butterflies hang like leaves from the Oyamel fir trees. These are impressive enough, but when the sun breaks through the normal heavy mist and warms the air sufficiently, the Monarchs begin to stir away from their roosts. The air fills with a cascade of golden butterflies, a truly moving and magical moment. If you stand motionless, butterflies likely will land on your head or hand.