WHAT CHUCK JUHN noticed most was the quietness. The road was full of people, but there were no voices. Hundreds of people—thousands—walked past him. This many voices would make a football stadium thunder. But they were silent.

They just walked, so many people that they filled the road, edge to edge, as far as Juhn could see, and all he heard was the soft, weary whisper of feet on the road. The people used every last scrap of energy just to keep moving.

"People at the margin don't mess around," remembered Juhn, an American aid worker who was in Rwanda at the time. "They didn't say a word. Total silence. They were so washed out and skinny; it was like some other reality moving through."

Juhn was watching one of the great single movements of people in recent years. It happened in December 1996. It was the sudden return of more than 450,000 Rwandan refugees from camps in Tanzania to their home country after the brutal upheavals of 1994 and '95. Forced out of Tanzania by government deadlines, the refugess left the huge camps in less than a week and started walking home.

"I'm glad I saw it," said Juhn, when I talked to him recently in Tanzania, "but I don't ever want to see something like that again."

He may not—movements of thousands of people seldom occur so suddenly, or so visibly. But something like this happnes every day. Behind the relatively stable appearance of much of the world, the same kinds of numbers are moving, almost as quietly. In airports and seaports and railway stations, along forested borders, and even where steel and barbed wire make barriers that seem impenetrable, thousands upon thousands of people are on their way to somewhere new. They don't make much noise, but they change the world.

Human migration: The term is vague. What people usually think of is the permanent movement of people from one home to another. More broadly, though, migration means all the ways—from the seasonal drift of agricultural workers within a country to the relocation of refugees from one country to another—in which people slake the fever or need to move.

Migration is big, dangerous, compelling. It's Exodus, Ulysses, the Battle of Agincourt, Viking ships on the high seas bound for Iceland, slave ships and civil war, the secret movement of Jewish refugees through occupied lands during World War II. It is 60 million Europeans leaving home from the 16th to the 20th centuries. It is some 15 million Hindus, Sikhs, and Muslims swept up in a tumultuous shuffle of citizens between India and Pakistan after the partition of the subcontinent in 1947.

Migration is the dynamic undertow of population change; everyone's solution, everyone's conflict. As the century turns, migration, with its inevitable economic and political turmoils, has been called "one of the greatest challenges of the coming century."

But it is much more than that. It is, as it has always been, the great adventure of human life. Migration helped create humans, drove us to conquer the planet, shaped our societies, and promises to reshape them again.

"YOU HAVE A HISTORY book written in your genes," said Spencer Wells. The book he's trying to read goes back to long before even the first word was written, and it is a story of migration. Wells, a tall, blond geneticist at Stanford University, spent the summer of 1998 exploring remote parts of Transcaucasia and Central Asia with three colleagues in a Land Rover, looking for drops of blood. In the blood, donated by the people he met, he will search for the story that genetic markers can tell of the long paths human life has taken across the Earth.

Genetic studies are the latest technique in a long effort of modern humans to find out where they have come from. But however the paths are traced, the basic story is simple: People have been moving since they were people. If early humans hadn't moved and intermingled as much as they did, they probably would have continued to evolve into different species. From beginnings in Africa, most researchers agree, groups of hunter-gatherers spread out, driven to the ends of the Earth.

To demographer Kingsley Davis, two things made migration happen: First, human beings, with their tools and language, could adapt to different conditions without having to wait for evolution to make them suitable for a new niche. Second, as populations grew, cultures began to differ, and inequalities developed between groups. The first factor gave us the keys to the door of any room on the planet; the other gave us reasons to use them.

Over the centuries, as agriculture spread across the planet, people moved toward places where metal was found and worked and to centers of commerce that then became cities. Those places were, in turn, invaded and overrun by people later generations called barbarians. The names of some of these groups have become symbols of upheaval: Hittites, Scythians, Visigoths, Vandals, and, lest we forget, Attila's vigorous warriors, the Huns.

In between these storm surges were steadier but similarly profound tides in which people moved out to colonize or were captured and brought in as slaves. For a while the population of Athens, that city of legendary enlightenment, was as much as 35 percent slaves.

"What strikes me is how important migration is as a cause and effect in the great world events," Mark J. Miller, co-author of The Age of Migration and a professor of political science at the University of Delaware, told me recently.

It is difficult to think of any great events that did not involve migration. Religions spawned pilgrims or settlers; wars drove refugees before them and made new land available for the conquerors; political upheavals displaced thousands or millions; economic innovations drew workers and entrepreneurs like magnets; environmental disasters like famine or disease pushed their bedraggled survivors anywhere they could replant hope.

"It's part of our nature, this movement," Miller said. "It's just a fact of the human condition:"

What drives this fact of nature, and how is it likely to change humans next? Over the past months I met a lot of people who were in various stages of migration. Without exception they were kind and gentle people. But the forces that drove them were ruthless.

"IT WAS DURING the nighttime when the soldiers came," said my interpreter, Muhammad. "They started to kill people; I mean shooting. And some of [the refugees] relatives were killed. Yes, some of their family were killed."

I was in an enclosure on the Tanzanian shore of Lake Tanganyika. The roof was a blue tarp, printed UNHCR: United Nations High Commissioner for Refugees. The UNHCR has programs in over a hundred countries and in 1997 had a role in the lives of 22 million people.

About 20 refugees from the small African nation of Burundi, which adjoins Tanzania to the northwest, sat inside. The day before they had come to this receiving center by boat down the coast of Lake Tanganyika. Among them was a refugee who told Muhammad in Swahili what had happened just a few days earlier.

He spoke impassively, as if explaining a machine. Beside him sat his wife and baby. He was 20; his wife was 18. The child was 9 months old and had sores all over his scalp.

"This man was with his parents there," Muhammad explained. "Up to this moment he does not know whether his parents are alive or if they were killed. For seven days they are walking, coming to this country."

I did not know what to say. The refugee looked at me without emotion, as if to say: This is too serious for sympathy. Twenty other pairs of eyes in the enclosure looked at me calmly and said the same thing.

"For you, shocking," said Muhammad. "But for us, not, because this we are hearing day and night."

A ledger kept here by the organization Chuck Juhn worked for, the International Rescue Committee, told a brutal story of the health of the incoming refugees: Malaria, dislocated shoulder, malaria, bullet wound, bullet, bullet, bullet, bullet, multiple bullet.

Scholars call this "forced migration." Most of the 20 million or more people a year who are involved with the UNHCR are fleeing from conflict. The number fluctuates widely. In 1991 it was 17 million; it grew to 27 million in 1995 and declined to 22 million in 1997.

These numbers are hard to visualize, but I will always find it very easy to see in my mind's eye the little family from Burundi sitting on a straw mat in the UNHCR reception center, waiting for a ration of high-energy biscuits and looking at me with that strange serenity, while Muhammad explained how they had kept themselves going, day after day, walking through the bush to get here:

"The hunger has been forcing them to walk," Muhammad said. &qout;Besides that—the peace."

A SIMPLE EXPLANATION for migration is that one place pulls on a person, with good wages, freedom, land, or peace, while the place in which the person lives pushes because of low incomes, repression, overcrowding, or war. Mark Miller urged me not to fall for such appealing simplicity. Migration, Miller argues, is not a matter of each individual deciding rationally and simply where the best hope for freedom or success lies. It is much more complicated and involves each person's history, beliefs, and family; his country's prior relationships with other nations; and the whole interlocking international web of existing migration routes and patterns.

However, push-and-pull factors, like the war in Burundi and the peace in Tanzania, are unquestionably part of the equation. I saw another part of the push and pull clearly in the United States and Mexico, where economics was hard at work shoving people around.

At first border tension in the U.S. appeared to me to be almost as deadly as the war in Burundi: Human beings look wrong when you find them lying in an alfalfa field facedown. They have the wrong dimensions; they look too thick. It is chilling to see them lying so still.

Luis Diaz, a U.S. Border Patrol watch commander, and I had found the bodies with the help of another agent, named Peter, who had a big night-vision scope mounted in the back of a pickup. By two-way radio he had guided Diaz and me across the field. "More to the east. Now south." Suddenly there the bodies were, lying facedown, looking like packages of hay wrapped in colored cloth, so much more still than they ought to be in the chilly damp moonlight of a California February night.

"Make a line!" Diaz said to the bodies in Spanish. (Border agents here seem to call all illegals "bodies," as in, "There's two bodies moving toward area five.")

"Hands on your heads!" Diaz went on. "Over to the car!" The bodies, which had looked entirely dead, all got up, brushed off dirt and leaves, and started walking.

&qout;How did you see us?" one of them asked in Spanish. Diaz didn't answer. "I'll let him figure it out," he told me later.

"Where're you going?" Diaz asked them.

<"Al otro lado," one said.

"Si," said Diaz, "but where on the other side?"

"Don't know," said one. "Los Angeles?"

"Is it beautiful, Los Angeles?" said another.

"Well," said Diaz, "it's big."

People from Central and South America call the United States El Norte, but to many Mexicans it is just over there, the other side. It sounds mystical. In a way, for Mexicans buried in poverty, it is.

"A better life," said Alejandro Bermudez Perez, when I asked him why he had tried to cross. He was in a holding tank at Luis Diaz's Calexico headquarters. There, at a desk surrounded by rooms that can hold a total of about 300 people, Alejandro was waiting to be fingerprinted and photographed by high-tech computer identification equipment.

He was a lean, quiet man of 24. He told me about his wife and child, who lived at the edge of Mexico City. He had come to the border by bus and wanted to get work in the U.S.

"Now that you've been caught, what will you do?" I asked.

Alejandro stared at me with his solemn young face, and looked as if he was going to break out laughing. What did I think, asking that question?

But he maintained a straight face; he wasn't about to tell me in front of an agent that he'd be coming right back at him tomorrow.

"Maybe," he said innocently, "now I'll go back to Mexico City."

Alejandro's choice to go north is a perfect example of economic migration. This form of movement has lots of numbers behind it. The World Bank estimates that only 15 percent of the world's almost 6 billion people live in the 22 highest income countries, where the average income is more than $25,000 a year. Nearly all the rest, as well as most of the 80 million or so people added to the world each year, live in countries in which income is close to or less than the global average of $5,000 a year.

This would seem to create a straightforward flow from poor to rich. But it is not that simple. Some 120 million people in the world live in countries other than the one in which they were born, but more than half of these have moved to other developing nations, where the average income is little better than the countries they left. In addition, though you'd think that the poorest people would be the ones most prone to migrate, that's not so either. The image of throngs of abject poor clamoring on the borders of the wealthiest nations is wrong.

"The world's poorest people don't move, for the most part," Mark Miller said. Though driven by need, the people who do move are usually those who can come up with a few extra dollars for the trip.

It's easy to see how a rich nation like the U.S. could get the wrong impression of its immigrants: On the U.S. side of the border, people like Alejandro look like criminals, climbing fences, slipping through the shadows, trying to evade the cops, hiding out in slums, ignorant of the language, dressed in clothes they've worn while lying in alfalfa fields. But at home, they're heroes.

ONE SATURDAY IN MARCH I took a green Volkswagen taxi out of downtown Mexico City to an area northwest of town, where dense waves of small concrete-block homes wash up into the hills, the edge of that city's rising tide of humanity. There, in a small room painted a sunny yellow, in which a patched aquarium gurgled on top of a refrigerator, I talked with Alejandro's family.

"When you have a son," his wife, Lourdes, said, "you don't think of yourself. You have to make sacrifices. To tell the truth, he knew it was going to be difficult."

Around the table were Alejandro's mother, a brother-in-law, and his younger brother, Cesar, who recently tried five times to cross to the other side and got caught each time. He's going to try again. "It's like a little worm inside," he said. "To go to the other side."

The family had heard from Alejandro. He'd called from California. After three or four attempts he had made it to the other side and was living with a few others who had crossed with him somewhere near San Diego. They didn't know how he was living, but I suspected it was in one of several infamously tough shanty campgrounds in the hills near the city.

The family had planned the trip seriously. In Mexico, Alejandro had been making 600 pesos a month (about $70) as a car mechanic; from that and with loans from relatives they had saved the equivalent of almost a thousand dollars, some to pay for his bus fare and a fee to a pollero—a guide to get him across the border—and some to keep Lourdes going until the dollars Alejandro made started to flow south.

The long-term goal was to save enough money so they could open a small business, probably a car repair shop, in Mexico City.

"That is our dream," Lourdes said. "To tell the truth, if he were just working here that dream could not come true. Never."

Wondering whether their hopes for Alejandro were romantic or realistic, I asked them if they knew many people who had come back rich enough to do what Lourdes hoped. "Si, some people," said Alejandro's brotherin-law. "But we also know people who have come back with nothing"

Before Alejandro left, Lourdes had small photographs of the three of them put in key chains; one for her, one for him, "for the times when he needs to encourage himself to go on," she said.

"To tell the truth," Lourdes went on, "after he left I worried about Alejandro every day. When he got there, I cried. Then I rested a little from that fear that I had:"

It was a pleasant morning, with warm air drifting through open doors and windows, a jacaranda tree blooming outside, voices of the children playing, and this competent and hopeful family talking about the loved one they'd launched into the distance. Watching them, I thought of Alejandro, living on the run in San Diego County, looking for any kind of work so he can send money home. Even when it's driven by economics instead of war, migration is still full of risks and hardship.

Alejandro's mother, Maria Elena Perez Ramirez, was no stranger to the hard realities of movement. She had been part of it. A worldwide flow has been moving to cities for years and has probably affected more people than any other form of migration. She moved to Mexico City from the countryside of the state of Guanajuato about 20 years ago, because poverty was even worse where she came from.

"I started the migration," she said with a laugh. "It is a chain."

UNLIKE THE FLIGHT of refugees, which is usually chaotic, economic movement is a chain that links the world. Migration, a significant part of the trend toward a global system of communications and economics, continues to push us toward change.

In the Philippines the culture is a buoyant chaos of Asian and Spanish influence from previous centuries busy with migration. Now this country sends its people off to work almost everywhere.

When I was in the Manila offices of the Overseas Workers Welfare Administration (OWWA), I saw stacks of brochures to be given out to prospective migrant workers. The brochures were remarkably frank.

If you go to Singapore, one brochure said, "Good behavior means that: The maid ... will not become pregnant." In Korea, another warned, "Employers are regarded as generally rude considering their strong voices and harsh ways. However, Filipinos ... should not take this personally."

If you're going to Hong Kong, said another brochure, "OWWA advises the [worker] to ... discard the `Filipino time' mentality or face the consequences." For Saudi Arabia the advice was succinct: "If you think you are embarking for paradise, forget it."

With a total population of 75.3 million people, the Philippines has 4.2 million working temporarily in more than 160 other countries, according to the OWWA. Of these, only 2.4 million are officially documented.

The Philippines experience demonstrates not only the global scope of migration; it also shows how far out on the edge of established society migrants can be. Countries that send migrants and those that receive them seem unable to deal with migration smoothly.

At home the Philippine government engages in complex and apparently contradictory practices. It does not officially encourage its people to work overseas, but it operates bureaucracies to help them go. Elaborate licensing systems, ostensibly for regulation, are actually designed to discourage riskier overseas employment, like maids or "entertainers"-who are often destined for what one Filipino official delicately called "the flesh trade."

The same kinds of contradictory rules prevail elsewhere. South Korea, for instance, calls its foreign workers trainees, though they may train at the same job for years at a time.

"A lot of countries are figuring out how to open the labor market [to outsiders] without appearing to do so," said Ricardo Casco, director of the marketing branch of the Philippine Overseas Employment Administration.

When I visited Ricardo's building, the place was full of people applying for work, taking orientation, even demonstrating dancing ability. Ricardo himself was an energetic, enthusiastic man who sounded like he would be at home on Wall Street, with talk of new paradigms, market behavior, and technology transfer. "

It's exciting," said Ricardo. "It's a historical, natural phenomenon to manage."

THE SONG was "Goodbye Yellow Brick Road," by Elton John. In the long room 11 young women danced solemnly to the tune, turning and bending in unison while several of their friends watched from a doorway.

"If she is beautiful," said Eric Tiu, watching thoughtfully, "she makes $1,500 a month." Tiu, a descendant of Chinese immigrants to the Philippines, is a recruiter of young female entertainers. He finds them on the island of Mindanao, 500 miles to the south of Manila. He brings them to the city, feeds and houses them for about four months, teaches them to dance, then sends them to Japan for three- or six-month tours, earning a fee from club owners for each one, as well as a hefty fee from the workers themselves. At the time I talked to him, he had about 700 entertainers in Japan and another 500 in training in Manila.

Tiu's staff, which includes some trained choreographers, teaches the women to dance because that's required by the government. Though everyone knows that these entertainers are hired to provide Japanese men with girls to flirt with, the government has decided to discourage the trade not with prohibitions but with regulations, and one of those is that each entertainer must pass a test of dancing skills she will hardly need.

Many of the dancers were cheerful. One was on her tenth trip to Japan; on the proceeds from her work she had bought a "jeepney," which is a kind of informal Philippine bustruck, and was building a house for her family. "This is part of our adventure," she said. But when I asked another entertainer if she would want a future daughter to take this kind of job, she threw up a hand as if to ward off the idea. "No." she said. "No!"

Many of the newer students looked scared and lonely. The dance floor was a poignant scene: the sound of the music, the dancers' eyes blank, their minds focused on the skills that would be so unimportant, and the middleaged entrepreneur looking at his precious cargo, calculating the dollar value of youth and loveliness.

Leaving the dancers, we walked upstairs to his office, where Tiu was installing a large Toshiba home-entertainment system. We passed one of the bedrooms and glanced in the open door. It was a small room, with several women sitting and lying around at one end of it, almost in a heap, reading and laughing.

"How many live in this room?" I asked him.

He shrugged. "Probably about 15," he said.

LATER IN TOKYO, I went to one of the clubs where Tiu's dancers worked. It was a low room in a basement, with streamers hung across the ceiling, a glitter ball rotating in the center, booths upholstered red velvet, and Philippine flags and sentimental paintings of Philippine scenes on the walls. The club was called Music Supper We Serve, but on the sandwich board outside were words in English: Philippine Club. The men didn't come in here for the Japanese booze.

The booths were full. There was a man in each, flanked by one or two Filipino women. They laughed and giggled together. Sometimes they got up and sang in time to a karaoke machine. All the people I talked with assured me that none of the women are expected to go home with the men but also acknowledge that sometimes they do.

Watching these women So far from home, I remembered what one recruiter (of domestics and factory workers) tells his clients to help them survive the first weeks on the job. "You will be so lonely," he says. "The only thing is to work yourself to sleep. And pray, pray, pray."

The money that entertainers and other working migrants send home is extraordinary. Young teachers in the Philippines make an average of $150 a month compared with entertainers' $500. The transfer of funds from migrants to their home countries, if lumped together, is one of the largest single movements of money in the world. In the Philippines it is an estimated eight billion dollars a year, almost three times the amount the Philippines receives in foreign aid. In all of Asia, money sent home by overseas workers totaled about 75 billion dollars in 1995, far more than the 54 billion dollars the same countries received in foreign aid.

None of the workers I met had signed papers saying they were going to return a single penny. Yet they did send home a vast amount of money, whole national defense budgets' worth of money, because of love.

MIGRATION TOUCHES raw places and deep places in many lives. One of those things, perhaps oddly, is the longing for home. This is one of the reasons immigrants so thoroughly enrich the cultures of the countries to which they move.

There's debate about what effect immigration has on the economies of the receiving countries. "Overall, society gains, but not everybody gains," says an economist at RAND, a Santa Monica, California, think tank. "People who directly compete with immigrants lose. The gains that do occur are smaller per person but affect a larger group of people." But there is no doubt that migrants' efforts to maintain a bit of home around them change the culture.

There are thousands of examples of this in the U.S. alone. One afternoon near Los Angeles I followed a group of Chinese lion dancers around a community. The dance was a raucous celebration that Chinese store owners greeted with little envelopes of money. And on another weekend morning I floated on an inlet in Long Beach with two Chinese dragon boats whose crews of immigrants and the very American-sounding children of immigrants were practicing for races later in the year.

These attempts to make the new place feel as comfortable as home don't always work. While 1 watched the dragon boats, a harbor patrol officer came over to tell the skippers that people in a condominium on the shore had complained of the drumming that set the pace for the paddlers. Since there were clamorous outboards racing up and down the same inlet and no one had complained of that noise, the message was clear to me: Someone did not like to have their view crossed by people from China.

Perhaps in part because of their awareness of an unwelcome mat, vast numbers of migrants, including many of Mexico's illegal visitors to the U.S. like Alejandro, never expect to move permanently. Though some labor migrants eventually adopt the new country, most return home. And among forced migrants—like the refugees in Tanzania—home is the most magical dream in the world.

THE BIG BOAT moved almost silently across Lake Tanganyika. It was after midnight. Burundi lay in distant, dark folds of mountain to starboard, and the Democratic Republic of the Congo, previously Zaire, was near at hand to port.

The land was dark, but the waters were alight. All around us floated what looked like candles, as if to show this vessel, burdened as it was with fears and hopes, the long path home. The lights were the fishing lamps of Congolese fishermen, who use the glow to draw fish up to their nets. The boat was a 180-foot ferry called the Mwongozo. The cargo was 800 refugees from the recent civil war in the Congo that had changed their country's name.

Like almost everything about migration, this journey was bittersweet and scary. Peace had been declared in the Congo, but it was fragile. The people on the boat, who had fled brutal attacks almost a year before, were not sure what they were going back to. Some of them told me their choice to sign up for this voluntary repatriation, sponsored by the UNHCR, was based simply on their hunger in the refugee camps, where rations had been temporarily cut in half because of washed-out roads and railroads.

"You might die in the Congo of a bullet," one refugee had told my wife, Suzanne, who speaks better French than I do. "But here your children will die of hunger." Yet the jam-packed boat was cheerful. People stood in a crush of lines to get heaping plates of rice and beans cooked by a company called Happy Caterers. Up near the bow, where belongings were piled high in tarpwrapped bundles, with a few handmade cages with chickens in them, Suzanne and I talked to a man named Jules, who had been a thirdyear medical student before the war chased him to Tanzania. With him was a friend, Patrick, who had been studying architecture.

Both were on board with their wives and kids. "When we left," Patrick said, "there was a war. Children were running away along the shores of the lake. But the war is over now, though we don't know if our houses are still there. I was happy to leave for Tanzania, but I'm very happy to go back now."

"At least now," Jules said, "we'll be able to work and to study."

"What's the first thing you're going to do?" Suzanne asked.

"Get something to eat," Patrick said.

In the middle of the night the boat anchored just off the Congolese shore, still accompanied by the lamps of the fishermen. When the anchor dropped, a cheer rose into the darkness. After the cheer faded, someone on the ferry gave a single whistle. It was like a long birdcall, testing the night for friends. From one of the candles on the lake, a whistle came back. Near the place the boat anchored to wait for the morning's disembarkation was a town named Baraka. I didn't know whether to take the word's meaning with irony or hope, but at least it reminded me of the way migration has woven its colors into every pattern of our lives. The word is Arabic, brought to the Congo long ago by traders. It means "blessing."

Learn more about human migration at www.nationalgeographic.com/features/2000/population/migration.