Some places the border is a muddy river, too thin to plow, too thick to drink. Other places it's just a line in the sand. Over the years mapmakers redrew it, wars moved it, nature yanked it all around as the course of the Rio Grande shifted. But what would it take to make it disappear altogether? If today is like any other day, this is what is going to come across the line from Mexico: a million barrels of crude oil, 432 tons of bell peppers, 238,000 light bulbs, 166 Volkswagen Beetles, 16,250 toasters, $51 million worth of auto parts, everything from the little plastic knob on the air conditioner to your cell-phone charger. It all comes in trucks and boxcars and little panel vans, and that's just the stuff that Customs can keep track of. There is also the vast shadow market, not just the cocaine and heroin and freshly laundered money, but cut-price Claritin and steroids and banned bug killers and boots made from the flippers of endangered sea turtles.

THE WAL-MART IN LAREDO, TEXAS (POP. 193,000), IS THE HIGHEST GROSSING PER SQ. FT. IN THE U.S.

And then there are the people, more than 800,000 crisscrossing legally every day, some walking, more driving, not to mention the 4,600 or so who hop the fence and get caught a few minutes or hours later. The ones who make it are on their way to jobs as meat-packers in Iowa and carpetmakers in Georgia and gardeners in Pennsylvania. They want to come here so badly, they will risk the scorpions and the rattlesnakes, the surveillance cameras and underground sensors; they will fold into hidden compartments in the dashboard of a car or in the belly of a tanker truck. They know they can get a job no one else wants, save some money, send some home, maybe find a way to bring their families—because someday, this border may not look anything like what it does now: a barbed-wire paradox, half pried open, half bolted closed.

So how much has to cross a border before it might as well not be there at all? There is no Customs station for customs—for ideas and tastes, stories and songs, values, instincts, attitudes, and none of that stops in El Paso or San Diego anymore. The Old World fades away—salsa is more popular than ketchup; Salma Hayek is bigger than Madonna—and the border is everywhere. One day soon it may seem a little backward not to speak some Spanish, even the hybrid Spanglish of the Southwest: "Como se llama your dog?" Signs appear in the store windows in Garden City, Kans., that say, se habla español, and you can buy extremely fresh mangoes at bodegas all over that town. Dalton, Ga. (pop. 27,900), has three Spanish-language newspapers. Says longtime resident Edwin Mitchell, 77: "We're a border community—1,000 miles away from the border." Already, we are living in a whole new world.

Sometime in the next few years, Mexico will pass Canada as America's top trading partner. Hispanics have overtaken African Americans as the country's largest minority, the swing vote to woo, the sleeping giant to waken. If Presidents George W. Bush and Vicente Fox manage to solve the problems of two countries that need each other but don't completely trust each other, the American Century could give way to the Century of the Americas, and the border might as well have disappeared altogether.

America's 4,000-mile border with Canada is basically defended by a couple of fire trucks, and most Americans think that's about all we need. The southern border is half as long, has the equivalent of an army division patrolling it, and many Americans say it should be buttoned down even tighter. At the beginning of a new century, there may be no country on earth with as much potential as Mexico to destabilize the U.S.—and to preserve its standard of living. No wonder people can't decide how much the border should be a barrier, how much a bridge.

From the moment you set foot in the boomtowns of the Rio Grande Valley, you sense you are watching the gold rush, headlong and free spirited and corrupt and ingenious. Stand on a corner some morning in Laredo, and watch the first of 8,000 trucks a day hauling the global economy north and south, 18-wheelers full of bulldozer claws and baby cribs, all passing through a town that once didn't bother to pave the streets. Now it can't pour concrete fast enough. The banks are open 7 to 7, seven days a week; the pager shops are everywhere. Every road is being widened, the road shoulders littered with pieces of blown-out tires.

Locals say you are not really a borderlander until your windshield has been broken at least once, from all the rocks flying out from under the big rigs. Much of the border is still desperately poor—McAllen, Texas, at the heart of the fourth fastest growing metro area in the U.S., is America's poorest city, the Commerce Department announced last month, with average per-capita income of $13,339 a year. But people on both sides are helping one another do the deals, cut the corners, take a region that was forever left behind and turn it into the New Frontier. The nafta prospectors saw in the opening of the border a chance to make a killing, take factories that would otherwise head to Malaysia and plunk them down right across the border, where the average Mexican worker earns slightly more in a day than an American makes in an hour, and where the highways run all the way to Canada.