People arrive at the Friendship Park in Tijuana, near the double steel fence that separates the city from San Diego. (Reuters)

A decade ago, when illegal immigration from Mexico was at an all-time high, this stretch of border was as good a place as any to sneak into the United States.

Migrants and smugglers could slip through the alfalfa fields outside town or plow their pickup trucks through the desert, where the biggest worries were stuck tires and getting safely across the irrigation canals.

But in the past five years, the international border here has become a harder, tougher, taller barrier — an American Great Wall. Miles of steel fencing now ride the desolate sand dunes west of Calexico. To the east, giant jack-shaped “Normandy” barriers, named for their resemblance to the defenses that once lined the beaches of northern France in World War II, block off old smuggling routes.

Overall, the United States has added 413 miles of new fencing to its southern boundary since 2006, raising to 649 miles the total length of border that has some form of man-made barrier to people or vehicles. The Rio Grande creates a natural partition along another 1,252 miles, and the government has been putting new fencing there, too.

Now the question is: How much more should be built?

Border Patrol officials say their current plans are to construct just one more mile of fence, in Texas. But as illegal immigration takes an increasingly central role in Republican campaign debates, several GOP candidates have renewed calls to fence the entire 1,969-mile boundary.

President Obama has made light of such proposals, saying fence advocates won’t be satisfied until the United States builds “a moat” stocked with “alligators.” But leading Republican candidates Mitt Romney and Newt Gingrich have vowed to barricade the entire U.S.-Mexico divide, with Gingrich signing a pledge to install a “double fence” while campaigning in Iowa earlier this month.

With such an endeavor projected to cost tens of billions of dollars, this stretch of California desert might be as good a place as any to assess how the existing border fence actually works.

The new barriers have been particularly effective at stopping vehicles from coming across, Border Patrol agents say. Along one stretch of desert here, the number of drive-through incursions plunged from 350 in 2007 to four so far this year.

But agents also say it is not the case that smugglers and illegal migrants on foot simply go to the place in the desert where the fence ends, and walk around it.

“Anywhere is a good place to sneak across if we’re not watching,” said Special Agent Jonathan Creiglow, a Border Patrol officer assigned to the agency’s El Centro sector here.

But there are also sections of 18-foot fencing right in the middle of downtown Calexico, opposite its sprawling sister city of Mexicali, where border jumpers can be up and over the wall in a matter of seconds, melting into shops and residential streets once they land on the other side.

At night, smugglers toss Hail Marys of pot-stuffed footballs and fling golf-ball-sized heroin nuggets over to waiting receivers. Stealthy ultra-light aircraft bomb the lettuce fields outside town with bundles of dope, then swoop back into Mexico, well below radar but high above the fence.

Then there are rugged sections in the desert where fencing is porous or nonexistent, but crossings rare. And those who do try to slip through are tracked by the Border Patrol’s growing array of sensors, high-powered night-vision cameras and surveillance drones.

In short, agents say, fencing is a tool and a first line of defense, but it does not bestow border security by its mere existence. “Without the fencing we wouldn’t have as much time, but nothing is going to stop them from going over or cutting through it,” explained Creiglow, who, at 26, is one of the many recent hires at the Border Patrol, which has doubled in size since 2002, with 18,500 of its 21,500 agents now deployed along the U.S.-Mexico frontier.

A costly barrier

Most of the barrier does not sit on the actual international boundary, but slightly north of it, allowing maintenance workers to access both sides without technically crossing into Mexico. Upkeep for the existing 649 miles of fencing is projected to cost $6.5 billion over the next 20 years, according to a 2009 report by the Government Accountability Office, and U.S. Homeland Security officials say the fence was breached 4,037 times in the government’s 2010 fiscal year, at an average cost of $1,800 per repair.

With most of the remaining unfenced stretch of border in Texas, the debate has shifted to the question of walling off the Rio Grande. Even in areas where the river can be shallow enough to wade across, putting a fence along the river’s sinuous levees is both costly and unpopular with local ranchers who want to preserve riparian access for thirsty cattle.

In Arizona, where Border Patrol agents catch more illegal migrants than anywhere else, lawmakers are soliciting public donations to put barriers along the remaining unfenced 82 miles of the state’s 370-mile boundary with Mexico. Such a structure would need to climb up and over steep mountain areas where construction costs are exorbitant and the deterrent value is questionable, enforcement experts say.

“I think the question is: What are you trying to achieve? Just to be able to say that you built a fence on top of a mountain?” said Thad Bingle, who was the Border Patrol’s chief of staff from 2007 to 2009. “If someone climbs 10,000 feet to the top of a mountain they aren’t going to be deterred by a 10-foot fence.”

Construction in rugged areas is made even more pricey because every stretch of new fence needs an accompanying road for maintenance and patrols, he added.

Fewer arrests

While the agency tallies the number of migrants it catches, it does not plot the locations of those apprehensions. But after hitting an all-time high of 1.6 million apprehensions in the government’s 2000 fiscal year, the number of arrests dropped to 327,577 in the 2011 period which ended Sept. 30, the lowest level since 1972.

Migration experts attribute the decline primarily to the weak U.S. job market — especially the lack of construction jobs — as well as growing fears of kidnapping gangs in northern Mexico. At the same time, average family sizes have fallen dramatically in Mexico, employment opportunities have improved, and the United States is letting more Mexicans in through the front door.

Mexican workers received 516,000 temporary work visas in 2010, “the highest number since the Bracero Program of the late 1950s,” said Douglas Massey, an expert on Mexican migration at Princeton University.

Tougher enforcement on the U.S. side has also been a factor, driving up the costs of getting across as well as the difficultly. But migrant smugglers on the Mexico side say the fence is hardly their biggest concern.

“There’s too much surveillance now,” said Luis, a husky guide-for-hire known as a pollero, standing in the Niños Heroes park in downtown Mexicali, where recent deportees and would-be border crossers gather. “The Migra [Border Patrol] has cameras everywhere,” he said.

Luis wouldn’t give his last name, but he said for $500 smugglers will get customers over the fence by creating elaborate diversions for the Border Patrol and deploying teams of helpers with roll-up ladders and ropes, even forming cheerleader-style human pyramids Better yet, Luis said, for $3,000 a guide will take you over the fence and through the desert at night, and $6,000 buys a legitimate U.S. visa rented from a look-alike with legal status.

“There’s always a way in,” he said with a wily grin.