When the sun comes out in the mountains of central Mexico, hundreds of millions of tiny, orange monarch butterflies fill the skies.
To see the monarchs in their element, we rode through the forest of Michoacán, more than 11,000 feet above sea level, with Homero Aridjis, who grew up in the mountains. Aridjis has been visiting the butterflies each winter for 60 years.
It's not just the sheer number of butterflies that is astonishing, it is the density of them in the landscape. It is as if the trees are covered in a bark of butterflies. Butterflies fall from the trees, landing on our arms and feet.
"They are welcoming us," Aridjis said.
Watch the story tonight on "Nightline" at 11:35 p.m. ET
Unfortunately, some of the residents of the region have not returned the welcome. Though monarchs fill the skies, they are facing a dire threat, because the forests where these wonders of nature make their winter home is disappearing, the trees being chopped down by illegal logging operations.
The people of this region have known about the butterflies for generations, but scientist weren't able to confirm until 1975 that these are the very same butterflies found in the Great Lake states and Canada in the spring and summer.
Researchers followed the monarchs on their long autumn journey to these mountain tops in the state of Michoacán. The butterflies will return north by spring, dying by the time they land in Texas. Over the summer, five generations will pass. How their offspring know to return these same trees on these same mountain tops is one of the great mysteries of nature.
The butterflies have been a major draw for tourists. Visitors from around the world trek up the mountains with guides to see the mystical, magical monarchs. Marcia Cebulska and Tom Prasch came here from Topeka, Kan., just to see the monarchs.
"It truly is magnificent to see the clustering and then when they come to life and the sun hits them its pretty amazing," Prasch said.
"When the sun hits them and they all take off -- chills," Cebulska said.
Trina and Richard Luther came from Mariposa, Calif., to take in this natural beauty.
"I never expected to hear the wings of the butterflies. How do I describe it? It won't fit in a camera, I can't capture it. It's something really marvelous to stand here and to see them flying off the trees, to see them flying into the air -- to hear their wings, it's amazing," Trina Luther said. "And to think that they travel from here to Canada -- without passport -- and the next generation returns, it's just something incredible."
The monarchs are drawn to Michoacán by the cool mountain air and by fir trees called oyamels, which are unique to this region. The butterflies perch on the oyamels' branches, wrapping their tiny feet on the trees' tiny needles. But the oyamel forests are under threat.
Since 2004, satellite images show that the forests have disappeared as a result of deforestation and logging. Farmland has crept up to the mountainsides, further shrinking the forests, and even though logging is illegal on the natural reserve, the forest slopes are littered with fallen trees and tree stumps.