While America continues to pour tax money into walling off the Mexican border, fully half of illegal aliens come into the U.S. by other routes -- many simply arrive via commercial jets, according to immigration authorities.

Expired student and tourist visas, phony marriages, unchecked container ships and the porous Canadian border to the north all contribute as much to illegal immigration as Mexican border jumpers, experts say.

That's especially true for those who mean us harm. According to the Center for Immigration Studies, 12 known al-Qaida operatives — including two conspirators in the first World Trade Center attack and four of the 9/11 terrorists — simply flew into the United States and were waved through customs on student or tourist visas. The terrorists later violated their vias by staying past the expiration date -- a practice that is not uncommon.

In fact, the Pew Hispanic Center estimated in 2006 that almost half of unauthorized immigrants, or up to 45 per cent, entered the U.S. with various kinds of visas allowing them to stay for a set periods of between 90 days and two years – and simply did not leave.

These about-to-be illegal aliens passed inspection at border crossing points in Mexico, some under the auspices of a legitimate Border Crossing Card, and processed through immigration checkpoints at airports, seaports or border crossings.

Pew estimates the total illegal alien population of 12 million is made up of between 4.5 million and 6 million "overstayers" entering on some form of legal permission.

Once they get it, it's unlikely that they will ever be thrown out, government officials concede.

In 2004, the General Accounting Office released a shocking report stating that the Department of Homeland Security hasn't got a clue as to how many "overstayers" are presently in the U.S.

That's because many border-crossers from Mexico and Canada are not required to sign immigration Form I-94 and, therefore, evade existing tracking systems.

Even if forms are properly filled out, the GAO noted, matching arrival and departure forms is a very shaky and easily perverted procedure.

"No data indicate how many overstay," the GAO report noted. "The extent of overstaying is significant and may be understated by DHS's most recent estimate."

A 2001 study discovered, for example, 6.5 million "apparent overstays" for which DHS could not match departure records with arrival records. This creates gaping holes in security against terrorism, the GAO reports.

The trend is hardly encouraging for the agencies involved in the expensive and sluggish effort to construct a 700-mile fence between the U.S. and Mexico. Congress has approved $1.2 billion to get the much-heralded border fence started, but government analysts predict the ultimate cost will be many times that amount.

"The fence is a joke," says Dr. George Weissinger, the former Immigration agent and associate professor at Briarcliffe College who wrote Law Enforcement And The INS. "It is a waste of money and a foolish enforcement measure, and it's not going to stop the illegal alien problem."

According to DHS, the number of illegal aliens in the U.S. jumped from 8.5 million to 11.6 million between 2000 and 2006. The GAO now estimates that another 500,000 illegals crash the border every year.

Weissinger estimates the number of illegals already here is much higher, perhaps as many as 20 million.

That number, he says, continues to swell as a result of "raft people" from Cuba or Haiti; drug smugglers who burrow mole-like under the southwest border in a maze of ingenious tunnels; and Asians who arrive by sea in cargo containers. All show the border fence up for what it is: a vastly expensive, politically wrangled, ineffective public relations ploy, he maintains.

"The Canadian border is more dangerous, because many Middle Eastern illegals come through there," Weissinger says. "People come in through every major airport and port of entry. Many enter legally with visas, but don't fulfill their contract by leaving when they say they will.

"At least half the illegal aliens in the U.S. came in legally through tourist or non-immigrant visas, but their intention was not to visit, but to stay," he says. "A border wall wouldn't have stopped them. Once you get into the U.S., it is unlikely that an immigration officer will apprehend you."

Then, there are the tunnels – dozens of them discovered in recent years, most constructed by drug smuggling cartels but equally effective as under-border routes for illegal aliens. The Los Angeles Times reported that seven of the larger tunnels, like the Grande Tunnel between Tijuana and San Diego, would cost $2.7 million to plug up.

The Congressional Research Service last year estimated construction of the 700-mile border fence could ultimately cost up to$1.3 million per mile, a figure that doesn't take land acquisition into account. Maintenance and repair could drive that cost to $8.3 million a mile — a whopping $5.8 billion — by 2016, the CRS reports.

The 700-mile fence would cover less than half of the 1,952-mile U.S.- Mexico border.

Still, the fence, if it ever fully comes to pass, would be formidable: one version is 16 feet high, 3-6 feet below ground, able to withstand a 10,000-pound vehicle crash at 40 miles an hour and resistant to vandalism.

To date, the DHS estimated that 150 miles of pedestrian fence and 115 miles of vehicle fence have been completed, and they hope to build 670 miles of total pedestrian and vehicle fence by the end of 2008.

But will it work? Gov. Bill Richardson of New Mexico doesn't think so.

"It's not going to work. If you're going to build a 10-foot wall, know what's going to come next? An 11-foot ladder," Richardson said

By: Clayton B. Reid