One sunny fall day in West Texas when I was a foolish boy, I caught hundreds of monarch butterflies and released them in my sister’s bedroom, where they landed in great masses on her flowered curtains. The visual impact of this sight was not lost on my mother, who nonetheless threatened to tan my hide if I didn’t release every one to the outside world. After opening the window and removing the screen, we watched the butterflies wing their way south. Where in the world, we wondered, were they going?

Thirty years later, I found out. As I stood atop a mountain at the El Roasrio butterfly preserve near Angangueo, Michoacán, Mexico, a dozen monarchs landed on my jacket and hat, and for a moment I looked like those flowered curtains. Then there was a sudden rustle in the tree above, followed by an explosion of orange and black, as thousands of butterflies took flight in the warming rays of the morning sun. All around me giant fir trees were covered with so many hundreds of thousands of butterflies that no green could be seen on the branches bending toward the ground. Everywhere I turned were butterflies and more butterflies. I gasped to catch my breath—not from the steep climb or the 10,000-foot elevation but from the exhilaration of seeing so much natural beauty.

Each winter more than 100 million monarch butterflies (as many as 4 million per acre) gather at overwintering sites in and around Michoacán, just a three-hour drive from Mexico City. El Rosario is one of several preserves owned by the Mexican government and the only one open to the public. If you have a long weekend to spare and want to experience the beauty that I did, make your travel plans now. The best time to see the monarchs is from mid-February to mid-March, when they mass in their densest congregations. Depending on when you go, you may even get to see their spectacular mating flight, as pairs couple in midair, the female’s wings folded as she dangles beneath the soaring male.

By the end of March, the butterflies depart on their journey north, fanning out over the eastern two thirds of the United States, from the slopes of the Rocky Mountains to the canyons of Wall Street and on into Canada. They follow a bountiful feast of flowers for distances hard to imagine. A hundred miles per day is not uncommon, even for these typically frail creatures; one tagged monarch once covered 286 miles in a 24-hour period. Along the way the females lay their eggs, primarily on milkweed plants, which provide a source of food for the rapidly growing caterpillar larvae. About a month later, a fully developed monarch emerges from its chrysalis and continues the migration. Before shorter days and cooler weather send the butterflies hurrying south again, three to five generations are born—a miraculous reproductive process that has been repeated annually for perhaps 40,000 years. Why do they always return to Mexico with such pinpoint accuracy? There is no proven scientific explanation, but one theory has it that the monarchs, like some birds, are sensitive to magnetism; thus they are attracted to the mountains there, which have large iron ore deposits.

Before you go, you might sign on with one of several package tours that serve the area, but you don’t have to: I found it a surprisingly easy trip to make on my own. I flew from San Antonio to Mexico City, where I rented a car and headed west on Highway 15. On the way into Toluca, I stopped to look at the huge statue of revolutionary hero Emiliano Zapata astride a mountainous bronze horse, then took the bypass around the south side of the city. (An argument for driving through Toluca is that you’ll get to stop at Casa de las Artesanias, or CasArt, a fine government-run crafts shop that has reasonably priced high-quality goods from all over the country. On Fridays, the city also has a crowded but colorful Indian market.) From Toluca, it was a beautiful one-hour drive through wooded mountains to Zitácuaro, a town of 67,000—though whether that number refers to people or speed bumps is anyone’s guess. Ten miles west of Zitácuaro, near the new Villa Monarca Inn, I took the well-marked right-hand turn up the flower-covered Angangueo valley to the mountain towns of Ocampo and Angangueo. Both towns are at the base of the El Rosario preserve, known officially as the Reserva Mariposa Monarca Santuario Sierra el Campanario.

The existence of this and other preserves is in part because of the work of the Mexican conservation group Monarca A.C. In 1986, reacting to extensive logging, Monarca prodded the government to ban all development in an area of 11,000 acres, to create more than 28,000 acres of buffer zones (with partial agricultural restrictions), and to purchase outright 2,000 acres of prime overwintering sites. These moves haven’t had exclusively positive consequences: In an effort to save the butterflies’ natural habitat, the conservationists displaced some jobs. But alternate sources of income for local residents have been found in such areas as farming and commercial chrysanthemum growing. Tourism has also been a money-maker; last year, 48,000 people paid ten pesos each (about $3) to visit El Rosario. “There’s no solid evidence that tourists are harmful to the monarchs,” says William Calvert, an Austin lepidopterist who discovered many of the overwintering sites in the mid-seventies. “It is habitat destruction that harms them.”

I chose to stay in the mining town of Angangueo—the heart of butterfly country—at the Hotel Don Bruno. The hotel was nearly full with an IMAX film crew, and no one wanted to talk about anything but monarchs. IMAX is a special camera process that uses 65mm film and a print ten times the size of regular 35mm film. Cinematographer Alex Phillips, who had done camera work on many movies for legendary director John Huston, told me he had shots of butterflies that would fill an entire 65-by 90-foot screen. “The movie is supposed to be about all of Mexico,” said Lorena Parlee, who was directing the film for the new IMAX theater at the children’s museum in Mexico City, “but after seeing the butterflies, we may rethink the rest.”

Many tourists in Angangueo pay $40 to hire a driver and vehicle to carry them up the mountain to El Rosario. Instead, I went unescorted. As I left pavement and eased onto an unlikely potholed alley outside Ocampo, two women waved at me and asked for a ride. They were Gina and Rosalinda, and they were on their way to the primary school in the village of El Rosario, where they were teachers. For half an hour, as I edged the car slowly up the beautiful mountain, driving through water at several crystal-clear stream crossings, Gina and I chatted in Spanish. Rosalinda, meanwhile, sat in the back seat singing happy songs about her son Chuy, who had a good job in Morelia and was going to marry a beautiful girl. I tried to ask Gina if she had heard an Indian legend about a young hunter who was told by a medicine man that if he caught the first monarch of the summer and rubbed the gold dust from its wings on his chest, he would become as swift and light as the butterfly. Unfortunately, my Spanish was not up to the story. She did tell me, however, that as a child she had heard that butterflies passing through the lower valleys around All Saints’ Day were said to be the souls of dead children on their journey to heaven.

Just outside of El Rosario, we were forced to halt because of a heavy rope stretched across the road. When I rolled down the window, a man trying his best to look official walked over and politely said that if we wanted to pass we had to pay him ten pesos. I was about to hand over the money when Rosalinda climbed out of the back seat, strode up to the man, gave him a stern lecture, and got back in the car. The rope was lowered, and we passed without paying.

After dropping the women at school, I continued up the rocky road to the parking lot at the sanctuary. A number of vendors’ shacks lined both sides of the path to the entrance—a veritable gauntlet of embroidered, painted, and carved butterfly memorabilia, as well as some tasty pork tacos. I paid my ten pesos and was assigned a knowledgeable, enthusiastic guide named Jaimé. (Though he did not speak English, some weekend guides do.) After signing the daily logbook at the small cabin that serves as El Rosario’s information center, I followed Jaimé up a well-worn path, first through thick stands of pines (which the monarchs do not care for) and then under a canopy of oyamel (the fir tree preferred by most choosy butterflies). Thankfully, steps have been cut into the steep path, though they do nothing to ease the sudden ache of heavy climbing in the thin air of that elevation.

Not far up the hill, Jaimé gently picked up a butterfly that had fallen to the ground and had frozen nearly stiff. “¿Está muerta?” I asked. “No,” he answered. “Fría.” Hoping to warm the monarch, I cupped it in my hand and started back up the hill. Within moments, it was stirring. By the way, it is an old wives’ tale that touching or rubbing the dust from a monarch’s wings will kill it. Lepidopterists tag monarch butterflies by rubbing off some of the tiny scales and attaching a tag to the edge of the wing. These tagging programs have shown us that real threats to the monarchs include cars; pesticides, which poison the water they drink; and herbicides, which indiscriminately destroy the milkweed on which the females lay their eggs.

As the first visitors up the mountain that morning, Jaimé and I came across hundreds of butterflies lying in the path. We placed most of them onto the adjacent shrubbery and dense flowers. If a butterfly falls to the ground on a cold night, it must climb back onto the low-growing flowers or bushes to escape the cold air. If those plants are trampled flat, butterflies on the ground have little or no chance of survival, which is why all visitors to El Rosario must stay on the clearly marked path.

Jaimé then showed me how to tell the females from the males (a distinct black dot marks each of the male’s posterior wings). As we moved on, I began to see large trees that seemed to have many frozen or dead branches. On closer inspection I realized that they were not dead needles but rather the brown undersides of thousands of wings. As the sun broke over the mountain and lit up the trees in gold and black, the monarchs warmed, filling the sky with their glorious flight. With one last warm breath on the monarch that I had carried up the mountain, I released it into the air. The butterfly, too, flew up into a blue sky that was ablaze in color.

My heart was greatly bolstered but my legs were feeling weak as we finally started down to the car. The butterflies were also beginning to move down the slopes to visit the wildflowers below. As we neared the bottom of the trail, I paused to listen to the whisper of thousands of butterfly wings that harmonized with the gurgling of a small, clear creek into a single sound; not water, not insect, but the sweet harmonious music of life.

Where to stay: The Hotel Don Bruno (011-52-725-80026), in Angangueo, is a lovely spotless inn just down the mountain from El Rosario. Call at least three weeks in advance to reserve a room with a fireplace; no matter what, be prepared for a cold night. Two other options within a few miles of the preserve are the Villa Monarca Inn (011-52-725-33346), on Highway 15 in San Felipe, which offers bungalows with fireplaces, and the Hotel Balneario (011-52-725-70056), which is in a tropical deciduous forest near Jungapeo.

Guided tours: Throughout February and March, Sanborn’s Viva Tours in McAllen (800-395-8482) has a seven-day package, including hotel, some meals, and travel in first-class motor coaches; the cost is $438 per person. From February 19 to March 24, Meta Butler Hunt Travel of Austin (800-759-1509) offers a trip that departs from Mexico City and is led by Austinite William Calvert; $1,350 per person includes hotel (double occupancy) and most meals. For $160 per person (minimum five persons), the Travel Institute of San Miguel de Allende and San Antonio (800-322-4888) runs a custom tour led by archaeologist Don Patterson. It leaves from the Mexico City airport and includes one night at a hotel.