It's 5 a.m., still dark, and there's a chill in the air. For Border Patrol agent John Tiltti, who works the Rio Grande, the game has been going on since midnight.

He knows that if he shines a flashlight near any of the many holes in the border fence, he will spot a few illegal immigrants. They know that if they run back to the river toward Mexico, he won't bother to pick them up.

By 7 a.m., the game begins to tilt in favor of the immigrants. At each hole in the "Tortilla Curtain," large groups of "illegals" have gathered along the river's concrete edge. Both sides realize that there is safety in numbers. If Tiltti comes near, the immigrants will do little more than back away. Some even smile and wave. He surveys them wearily and drives his patrol car on.

"Most of them cross every day. It's a game to them," he says.

This is a community of more than a million Hispanics, with a dry ditch in the middle. On one side of the ditch is El Paso, U.S.A.; on the other, Juarez, Mexico. Once the Rio Grande truly was grande; now in places it is just ankle-deep. If the immigrants don't want to get their feet wet, they can cross a bridge or pay enterprising Mexicans a dollar apiece to be carried piggyback through the water.

El Paso starts out rich at the northern end and gets poorer toward the border; Juarez starts poor at the river and gets even poorer further south. So there is little to distinguish the bustling but sleazy areas of southern El Paso from their Juarez counterparts across the river. It looks like one community, and many Juarez residents treat it as such.

They sleep in Juarez and work in El Paso. If they don't have a border-crossing pass, they commute across the river. If someone like Tiltti catches them and returns them to Mexico, they'll be back in hours.

Added to them are the Mexicans and other Latin Americans who plan to make the United States their home. They are more likely to head for the interior and settle in a city with a large Hispanic community such as Los Angeles, Denver, Dallas or New York.

This is the immigration problem with which Congress is attempting to deal through the Simpson-Mazzoli bill, which would legalize the residency of some aliens and seek to closely monitor immigrant employment.

Last year, the Border Patrol apprehended more than 1 million illegal aliens, and nobody knows how many more slipped through. Many believe that the deterrent of the Border Patrol will never be enough to counteract the lure of jobs.

"You can earn as much in a day here as in two weeks over there," says Tiltti, pointing to Mexico.

Rep. Ronald Coleman (D-Tex.), who represents the El Paso area, says, "One of the true monuments to the ignorance of mankind is the suggestion that you can place barriers across places where people want to travel."

The Border Patrol is well aware of this, but there is little it can do. Larry Richardson, chief patrol agent at the El Paso station, is philosophical: "All we do is send them back to 'Go' like a big Monopoly game," he says.

Tiltti, the son of Finnish immigrants, has been on the job for 15 years. Before that, he drove a truck in New York. "I like to drive," he explains dryly.

It's an understatement. He tackles the dirt levee roads in the dark without his lights, as if he were on a motorbike scrambling course. While he takes the corners at stomach-wrenching speed, he is apt to open the driver's door and spit out tobacco juice. Not for nothing is he known by the other agents as the Flying Finn.

By dawn, the immigrants are coming over thick and fast.

"There's about half a dozen crossing Paisano and another half a dozen going west down the access road," Tiltti radios headquarters.

"It's no problem to catch aliens -- they're all over the place," he explains. "It's no mystery. We know where they're crossing and where they're going. It's just a matter of how many people we have working that day as to how many we catch."

Most days, the El Paso patrol picks up between 600 and 800 aliens. Tiltti estimates that this is about 20 percent of those crossing each day. Richardson is slightly more optimistic; he puts it closer to 1 in 3. But it's difficult to calculate and complicated by the fact that the same person may be apprehended twice or even three times a day.

If they are taken in by an agent, they will fill in a form and maybe spend an hour or two behind bars in the office before being packed off to Mexico. Generally they go before an immigration judge only if they choose to or if they have committed a crime.

The agents are courteous and friendly to the people they catch this morning. "Goodbye, thank you," one young man says as he gets out of Tiltti's car to be herded into a van. "It's nothing, Tiltti replies, using the Spanish "de nada."

Stationed in a parking lot on the hillside, with a good view of the river, Tiltti surveys the scene through a pair of high-powered binoculars. "There's just too many of them and not enough of us," he muses. "There'll be 40 or 50 coming in at the same time. If I was there, I'd only catch five or six. The best you can hope for is that they run south back to Mexico. Then at least they didn't get in while you were there."

Does it depress him to be so ineffective? You learn to live with it," he says. "You do what you can and that's about it."

It is hard to avoid cynicism. "Every now and then, I get bored with it -- the same old faces, the same old game. If you put them in jail occasionally, they know it's not a game -- we do it just to let them know we're still in control."

But, apparently, they're not in control.

"I know," he admits, "but we don't want to let them think that."

From a helicopter, the contrast between the two countries becomes much starker. On one side of the river is a dry and dusty brown hillside, dotted with ramshackle one- or two-room hovels. The roofs are disintegrating, the roads are dirt tracks and there is little evidence of running water or electricity.

Down to the right are prosperous brick houses, tarmac roads and well-tended lawns. Not just that: a country club with a lush green golf course, lakes, tennis courts and swimming pool.

The border, dividing the First and Third worlds, must be the only one of its kind in the world.

The helicopter veers left to survey the sand hills. "It's desert from here to California," says pilot Bill Green. The boundary fence in the desert consists of small stakes with three or four strands of wire stretched across. There are no guards.

Green's job is to look for tracks of border-crossers and to alert ground patrol, but it is already 7 a.m. "By the time we pick up tracks at this time in the morning, they're long gone. They're probably working in the fields by now," he says.

The sand hill team on the ground, which tries to stop people crossing the desert, also uses underground sensors like those used during the Vietnam war. These detect vibrations on the surface, and the order in which the sensors go off tells the sand hill team where the desert-walkers are.

Catching border-crossers in the desert has higher priority than stemming the flow across the river. Given its limited resources, the Border Patrol has chosen to concentrate on preventing two things: the smuggling of people across the border for money and the influx of aliens to the interior.

Smugglers will often act as "coyotes," leading the immigrants -- often for days -- across the desert. "If you're talking about going to the interior and don't want to have to ride the freight trains or hitchhike, the chances are you'll have to use an organized smuggling ring," says Richardson. That can be expensive: "You can talk thousands of dollars apiece, and when they handle hundreds of thousands of people, that's a lot of money."

Getting out of El Paso is a lot harder than crossing the border, but the chances are still about 50-50, according to Border Patrol estimates. The patrol has checkpoints on all major highways leaving town, and at the airport plainclothes agents sometimes ask for ID.

But it is the freight trains that are probably most popular. There are three rail lines out: the Santa Fe, the Southern Pacific and the Missouri Pacific. "If they want to get on a train going out of El Paso, they have no problem finding one," says Robert Cranston, who supervises the train and sand hill checks.

Getting into the freight yard is no problem either. "Here are the railroad tracks," Tiltti pointed out as he hurtled down the levee road. "They're right next to the river. They're a main line to California and the river is just a hop, skip and a jump down there."

Groups of agents work the yards most of the day and night. They try to concentrate on trains going north; those heading east or west will be checked again by agents in other areas.

In the desert, Cranston checks the sand hills for footprints as a train sidles round the mountain. As it passes, he thinks he spots someone peeping out of a hole in one of the cars. Luckily, the train has to stop for a signal. Cranston and another agent leap from car to car, looking in the usual hiding places. All they find is a food bag; the immigrant must have jumped off and hidden in the bushes.

Despite the searing midday sun, the agents are methodical. They start at each end of the train, which must be half a mile long, peering underneath and on top of the cars. It is a scene reminiscent of World War II movies, when escaped prisoners of war hiding in cargoes of grain or inside crates hold their breath as soldiers walk past.

"They'll ride in the most dangerous places," Cranston says. "They'll get under the freight car and cling on." Is it worth it? "If they can get out of El Paso, they've got it pretty well made. It's the price you pay for the good life, I guess."

Back at the freight yard, the agents have found 39 aliens on a departing train. On its way out of the yard, the agents try again. One man stands on a platform about 15 feet high to look down into cars. Others jump deftly from car to car, hauling people out of the most unlikely places. Another eight are caught, one a girl who has tried this three days in a row.

Meanwhile, on the Carlsbad highway checkpoint, a half-hour drive out of town, agents Glen Wood and Ray Powers are stopping vehicles. They wave some through. They ask the occupants of others if they are American citizens. If the answer is a confident yes, there are no further questions.

"You go by a person's demeanor -- how they look, how they dress, whether they have accents," Wood explains. "If people look bewildered, I'll ask for their papers -- also if the car looks low to the ground or there is stuff in the back seat that should be in the trunk."

If you have blond hair and blue eyes, you are likely to be waved through with no questions asked. "It's a touchy issue," Richardson admits. "We're second-guessed all the time on actions that might conceivably be based on racial appearance. We try to get people to understand -- the reason for apprehending someone is that you suspect he's violated the immigration law, not because he's Mexican."

He cites the 90 nationalities of illegals apprehended in his area last year, but agrees that the total figures are against him: more than 200,000 were Mexican; 2,000 from other countries.

The non-Mexican illegals are the most exploited by smugglers, according to Oscar Martinez. He heads SIBA -- Stop Interior-Bound Aliens -- a nine-agent group set up four months ago to try to catch the smuggling rackets that operate out of El Paso. There is also an Anti-Smuggling Unit, which focuses on infiltration and intelligence.

Salvadorans, Nicaraguans or Guatemalans are more likely to pay a smuggler because they don't know how the border works, they want to ensure that they reach the interior and they have more to lose if caught. The smugglers cram them into trucks, vans and even motor homes. Martinez, who works nights, patrols the common loading areas in his unmarked car, keeping the radio out of sight. If he spots a suspicious vehicle, he follows it discreetly and calls in other agents to help.

He says a Mexican would have to pay $300 to $500 to be smuggled door-to-door to somewhere like Denver. For an illegal from another country, the fee would be $1,200 to $2,000. A truckload of 30 Salvadorans could net a smuggler as much as $60,000.

"Alien smugglers don't have much concern for the people they're smuggling," Martinez says. "They look upon them as merchandise." He cites drivers jumping out of vehicles while being chased by SIBA agents, leaving the trucks to careen toward the river or a tree.

"I think these people are worse than drug smugglers," he says vehemently, and adds, "Smuggling is such a lucrative business and the penalties are so minimal. The risks can be taken. The profit is there and the punishment isn't."

William E. Weinert, an immigration judge in El Paso, agrees: "Smuggling is big now, and it's profitable. You can make almost as much smuggling aliens as you can smuggling dope, and if you do get caught, it's next to nothing compared with narcotics." Penalties vary from light suspended sentences to a few years in jail.

"A lot of people are weekend smugglers," Martinez says. "They come down here from somewhere like Denver on Thursday, recruit in Juarez on Friday, and smuggle the immigrants over on Saturday night. They're in Denver by Sunday evening and back in their jobs by Monday morning."