Pete Moran crouches beside a patch of soft sand and runs his fingers along the knobby tracks in it. ''These are from kids in that house over there,'' says the US Border Patrol agent, pointing at the curving treads of a child's bicycle and the sneaker prints that follow them.

What we're looking for, however, are signs of illegal aliens, who sometimes use this country lane to slip over the border into the United States.

And they're coming at an ever-faster pace.

According to US immigration officials, the number of arrests at or along the border has risen from 10,839 in 1980 to 14,643 last year. And these officials estimate that they detect only about 10 percent of those who cross illegally.

Some of the activity is for profit, with smugglers earning about $2,000 a head to transport job-seeking Latin Americans to New York City. Chinese, because of the longer distances involved and a larger and more involved smuggling network, may have to pay more than $5,000 for the same service.

But much of the smuggling is done for free - by friends or relatives. An uncle brings a nephew across by crowding the whole family into the car and hoping customs officials at the border don't ask too many questions.

''Most of the smuggling is done with phony documents right through the ports of entry,'' says William Truesdale, deputy director of the US Immigration and Naturalization Service (INS) in Buffalo, N.Y.

Identification papers, which can be bought in Montreal or Toronto with relative ease, include everything from fake passports to New York City voter-registration cards to Puerto Rican birth certificates.

But while smuggling is on the increase in the Northeast, the federal government is shifting money and manpower for enforcement toward the more troublesome southern border with Mexico.

At the same time, the mood in the US appears to be growing in favor of tougher immigration policies. For example, the widely publicized Simpson-Mazzoli bill is expected to reach the floor of the US House of Representatives later this month. It would, among other things, use sanctions against US employers to make it tougher for illegals to hold jobs here.

The number of illegal aliens coming from Canada are small compared with those who cross from Mexico. In an average week more illegal aliens are caught at the Mexican border than were snared by the Border Patrol along the Canadian border in all of 1982.

With their resources stretched thin, immigration officials here are focusing on shutting down the smuggling rings.

At least 11 Toronto-based operations are under investigation by US authorities. Twice that number are thought to be smuggling aliens through the city.

The US-Canadian border - spanning more than 4,000 miles from ocean to ocean - has traditionally been one of the most loosely guarded frontiers in the world. On the back roads that link southern Quebec with northern New England alone there are plenty of crossings where guard posts either don't exist or are manned only part time.

Actually, it's not illegal to enter either country on an unguarded road. Anyone who does is just supposed to make a beeline to the nearest customs checkpoint before venturing farther.

But from one unguarded back road near this small Vermont town a smuggler could choose any of five different routes to the main highway - bad news for a Border Patrol agent trying to tail a suspect. Just a few miles farther in either direction are I-89 and I-91, both offering straight shots to Manhattan or Boston without a single stoplight.

''Unless you get them right off the bat, they can just disappear,'' says agent Moran, who patrols this 20-mile stretch of border. It includes eight open roads, only two of which are guarded around the clock.

Still, the border is more secure than one might think, agents say - especially in New England and in New York State. In the 300-mile stretch known as the Swanton sector, hidden electronic sensors help keep tabs on the traffic.

When Moran kneeled to check the sand at one unguarded border crossing, a light began flashing on a Plexiglass schematic board at Border Patrol headquarters more than 10 miles away. Within moments, the radio in his patrol car crackled with a cryptic message: He had tripped the alarm himself.

There are other methods of bugging the border, too. Sensors are planted where agents suspect smuggling activity, and they're relocated regularly. And otherwise unguarded roads often have alarms embedded in the tarmac so officials back at headquarters can tell which direction a car or truck is moving.

Residents along the border also aid in the enforcement effort - often acting as informants for the Border Patrol. Says Moran, easing the patrol car back onto the highway, ''If one of these farmers spots a crowd hiking across his back 40, he usually gives us a call.''

The Swanton sector is a hot spot for alien smuggling. Last year more than 2, 000 illegal aliens were apprehended here. They represented 67 different nationalities, including one man from Upper Volta.

Every day, more than 1,000 commercial trucks rumble across the Peace Bridge and into Buffalo from Fort Erie, Ont. Most of them cruise through customs with only a cursory check.

This year, though, immigration officials at the bridge have been finding illegal aliens stowed in compartments and under the tarpaulins of tractor-trailers - a sure sign, they say, that smuggling operations are becoming larger and more sophisticated.

''We're seeing even more activity now than we did during the first half of the year,'' says Mr. Truesdale. The record haul so far: 20 aliens stowed on one truck.

Most of the known smuggling rings specialize in a single nationality. The biggest operations in Toronto deal in Guyanese, Chinese, and Jamaicans. A typical large-scale operation might move 100 aliens a month. Smuggling arrangements often are made by relatives living in Canada or the US, say those who are familiar with the practice, although some rings are known to recruit aliens in their home countries.

Usually, an alien takes a commercial airliner to Montreal or Toronto, where he is met by a smuggler. The smuggler takes him to a ''safe home'' somewhere in Canada.

From there, the alien will be brought across the border by any of a number of techniques. He may ride in the trunk of a car, hidden on a tractor-trailer, or be dropped off along a secluded border road and then picked up on the other side after walking across alone.

The relatively short northern summers tend to make smuggling a fair-weather activity. Some aliens have become lost and frozen to death trying to cross the border in snowy weather.

''They've usually been told their chances of not getting caught are quite good,'' says Buffalo attorney William Reich, who has represented smugglers caught on the Canadian border. In some cases, the aliens pay for the service only after they're safely across the border - a sort of guaranteed delivery policy.

Mr. Reich, however, disagrees that the Canadian smuggling rings have become sophisticated.

''If this were really an organized criminal activity,'' he says, ''I'm sure they'd think of renting the cars and trucks they use.'' Instead, many of his clients have had their personal vehicles seized by the INS.

Most illegal aliens end up in New York, Boston, or Philadelphia, where they join relatives or friends who arranged the smuggling. Many take low-paying jobs in garment-district sweatshops or ethnic restaurants.

Some smugglers offer various types of package deals, sometimes helping aliens find places to live and their first jobs at the US end of the pipeline.

''The aliens sometimes delude themselves into thinking the smugglers are their friends,'' says Truesdale. ''But these guys are in it strictly for the money.'' After they've taken jobs, he adds, the illegals are sometimes blackmailed by smugglers who threaten to turn them over to the INS.

Other smuggling operations plug directly into illegal marriage rings operating in East Coast cities, according to Robert Hurley, supervisory criminal investigator with the INS in Boston. In addition to paying the smugglers, the alien using this method will frequently pay as much as $2,500 for an American spouse - a relatively easy way of gaining legal immigrant status, he says. But the ''spouse'' often is just a stand-in, hired to play the part using falsified or stolen identification.

Immigration lawyers agree that aliens use Canada as a steppingstone because its immigration laws are relatively lenient.

More than 70 nationalities can enter Canada without visas. Travelers to the US from other countries, by comparison, usually need to show valid visas just to get on a plane. Canada does require each visitor to show a letter of invitation from a Canadian, but only once he has landed there.

''We'll frequently call the people who authored the letters,'' says Allan Highfield, district administrator for Canada's immigration office in Hamilton, Ont. ''But they're usually in on it too.''

In the last five years, Canada has tightened its policy, adding such countries as India, Pakistan, and Chile to the list of nationalities requiring visas. Other countries are likely to be added to the list this fall, says Mr. Highfield.

Canada also has begun clamping tighter security on the border. Earlier this year, the Royal Canadian Mounted Police established a division devoted to patrolling the border. Before that, patrolling had been only an informal function.

Edel Estrada was the first of a dozen Cubans who defected when his plane stopped in Montreal on its way to Czechoslovakia four years ago. He, like some of the others, was with a troupe of entertainers.

Five of the 12 were members of the same family. They were met in the airline terminal by a priest. But Edel, chuckling at the memory, says the ''priest'' was really a physician from Miami and a friend of the family. Within days, the five ''disappeared'' with their friend - presumably to Florida.

Three other defectors had relatives in New Jersey. The relatives drove to Canada for a visit - and the defectors vanished with them.

A ninth member of the group, a ballet dancer, flew to Halifax, Nova Scotia, where he was able to slip onto a Boston-bound flight. Once inside Logan Airport at Boston, he requested political asylum in the US. The 10th and 11th defectors eventually followed via the Halifax route.

After five months in Canada, Mr. Estrada was the last of the original dozen still in Montreal.

''The last ones kept writing to me, saying, 'Go through Halifax.' But I was afraid: Too many people had already done it, and I thought they'd be watching,'' he says.

Finally, faced with language problems in French-speaking Montreal and uncertain about his immigrant status in Canada, Estrada decided to follow his compatriots. He had his girlfriend drive him to Rouse's Point, N.Y., where US customs officials asked to see her driver's license.

''We said we were going for a long weekend in Boston,'' says Estrada. ''When the guards asked if I was Canadian, all I had to say was 'oui.' It was unbelievable how easy it all was.''

Estrada is in this country ''under color of law'' - which means he doesn't have status as a legal immigrant, yet he's here with the ''knowledge and permission'' of US authorities. He requested political asylum when he arrived in Boston and was given permission to work while his case was considered.

Last summer his request was finally denied. The INS told him he didn't have a well-founded fear of persecution if he were to return to Cuba, even though he lost his job as an actor with a theater company in 1972, when the Castro regime deemed him of ''doubtful political and moral stature.'' He is appealing the INS decision.

Still, the Estrada case illustrates the foot-in-the-door approach to immigration: Get into the US first and worry about the paper work later.

According to immigration lawyers interviewed by the Monitor, US laws tend to foster this attitude. There are 33 different reasons someone can be ''excluded'' from the US at the border, among them having physical impairments, being addicted to drugs, or holding Nazi sympathies.

But once someone gets into this country, he can be removed only through expulsion - a complicated legal maneuver that can lead to years of appeals and petitions.

About one-third of those nabbed trying to enter the US illegally at the northern border are Canadians.

Most come south looking for work, particularly in the construction trades. Canada's economy, which was until recently mired in a deeper recession than that in the US, has exacerbated the problem.

The Canadians usually enter the US in their own cars, which are sometimes registered both here and in their own country.

Typically, they claim to be temporary visitors, then stop a few miles after crossing the border and attach domestic US license plates to their vehicles. That way, they reason, they'll be less conspicious.

The stumbling point is when their cars are stopped and searched by suspicious border guards.

''If a guy is 21 years old, unemployed in Canada, with a trunk full of tools and only $250 in his pocket, he's not going to Florida just to visit for the winter,'' says Moran. ''But you need to have fairly concrete proof to keep a Canadian out of this country.''

But occasionally breaches of the frontier are far more innocent. Canadians drive into the US on open roads without realizing they've crossed. Americans make the same mistake going north.

''We give them three miles to notice that the signs are all in English and there are American flags everywhere,'' says Peter Eaton, a Border Patrolman in Derby Line, Vt., who says he has chased more than a few lost Canadians.

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