Reporter's Notebook by GINGER ZEE

I've always loved butterflies - so fragile, graceful, and seemingly so rare.  Growing up in West Michigan, there were a few months each summer where you would see monarchs floating through the sky, but I never imagined seeing millions in one place.

While traveling to cover the World Cup of paragliding in Valle de Bravo, Mexico, I was happily informed that if we took a short drive we would run into one of the largest monarch sanctuaries in central Mexico. (The gliders say there is something magical about this place and love it as much as the butterflies do.)

The fact that monarchs have settled in central Mexico, of all places, is simply fascinating.  The monarch generation is born in North America (mostly Canada), flies south more than 3,000 miles, traveling 50 to 100 miles per day, to spend the winter in the shrinking forests west of Mexico City. Each year, they fly to the same place as they have for thousands of years.

A generation of monarchs lives between seven and eight months, depending on gender. Males mate with females (four to five times per day, every day for weeks at a time), then die. The females travel on with their fertilized eggs to the southern U.S.  From there, they live four to five weeks, mate with males who then die, and the females are off to the central U.S. They follow this same process for at least two more generations until they reach the northern portions of the U.S. or Canada.  Then the great migration begins again with all of these creatures filtering down to central Mexico.

Our trip was much easier: a five hour plane ride from New York City and then a 100-mile drive west of Mexico City to the sanctuary.

As we approached Piedra Herrada, our cab driver had to slow down under 5 mph.  There are laws in this region about going too fast because butterflies swarm the roads and you can kill them. Butterflies inundated the street, hampering our visibility as they flowed down from the top of the mountain to the creek for a drink.

Slowly, we made it through the road and arrived at the sanctuary base, where we met up with a guide and horses to take us on the 45-minute trek up the mountain. It's nearly 10,000 feet to the top. With about 200 yards to go, we had to leave the horses behind. Monarchs detest the smell of horses and dogs, according to our guide, so we hiked up to the roped-off sanctuary where millions of butterflies huddled together.

(How do they know there are millions? Scientists use plastic chips and tracking devices to follow their paths and then estimate using computer modeling.)

In a huddle in the trees, the butterflies made the branches look like heavy, moss-laden limbs.  The guide told us that sometimes the weight of all the featherweight butterflies combined proves to be too much to bear and the branch will break.

The butterflies huddle to preserve heat, especially when the sun is obscured by clouds, as it was during our visit.

The pure beauty of the spot becomes eerie when you think about the numbers.  Monarchs are measured by number per hectacre.  The lowest population recorded to date was in 2009 with 1.92 hectacres.  The largest population was about 20 hectacres in 1996 - that's as many as 1 billion monarchs. 

The 2011 numbers aren't in yet, but Chip Taylor, one of the scientists dedicated to studying this fragile insect, worried the numbers will be just as low as 2009.

What's wrong?  Deforestation and development.

"We lose 2.2 million acres a year; that's an astounding amount of habitat to turn into concrete," Taylor said in a 2010 interview with National Geographic. "You can't just keep on doing that without having significant impact on wildlife out there."

Environmentalists have gone to great lengths to keep the sanctuaries protected, but deforestation remains a serious issue. Development in Mexico and the U.S. has also depleted the milkweed plant, which monarchs use to lay their eggs. Without it, they are not able to reproduce. ( Find out more on that here.)

Exploring the sanctuary, it can feel eerie at times with countless dead butterflies laying on the ground. The guide told us they help the new generation find these places every year, to begin their journey again as generations have for thousands of years.