Today, optimistic Mexican illegal immigrants traipse across the U.S. border; last month, authorities discovered a 2,400 yard tunnel from Tijuana, Mexico into southern California being used to transport tons of drugs.

Tomorrow, devious al-Qaeda terrorists sneak into our southern states, and an underground-tunnel from Mexico into the U.S. transports a nuclear weapon enroute to a major U.S. city.

Regretfully, this speculative scenario is more realistic than Americans are willing to admit.

Illegal immigration and homeland security are intimately entangled, and the U.S. is vulnerable on every front. Yet, as Americans, we struggle to uphold our historically pro-immigrant principles while striving to protect our citizens’ lives, resulting in stalemated opportunity for actionable policies to solve this national security dilemma.

Terrorists can be counted on to try to exploit this vulnerability.

Over 500 million people cross our borders each year, and only 170 million are American citizens. Specifically, the Mexican border sees 10,000 illegal alien crossings each day – that is more than three million illegal attempts each year. The U.S. illegal immigrant population has swelled by an estimated 3.7 million over the last five years.

Lately, Mexican officials considered distributing maps of the border terrain, cell-phone coverage on the trails, and water stations maintained by the U.S. charity Humane Borders to help promote safe (illegal) passage. However, Mexican officials say the government wants to ‘rethink’ this plan because they are concerned the maps would direct anti-immigrant groups (like the Minutemen civilian patrols) precisely to where migrants would likely gather -- thereby ostensibly guaranteeing capture.

It should be noted that Mexican President Vicente Fox has a very clear incentive for aiding illegal migrants into our country -- migrant workers in the U.S. send home an estimated $16 billion a year. The revenue is Mexico’s second largest source of foreign currency after oil experts. President Fox has made it all too clear that he wants our shared border porous and will continue to pressure the U.S. to keep it that way.

However, those crossing the borders are not only Mexican drug-traffickers, optimistic workers, or families. More than 100,000 illegal immigrants entering the U.S. from Mexico each year are not Mexicans. Last year alone, 450 people crossing the Mexican border were from such officially-designated “special interest” countries as: Afghanistan, Angola, Jordan, Qatar, Pakistan and Yemen.

As for our northern neighbors, the vast Canadian border has also been exposed as a weakness. In 1999, Ahmed Ressam was arrested entering Washington State with explosives and bomb components in the trunk of his car. That same week, authorities thwarted plans by Lucia Garofalo and Bouabide Chamchi to terrorize millennium New Year celebrations in the United States when they arrested the pair on the Vermont-Canada border.

While we do not know yet whether the southern border has been breached by terrorists to gain entry into the U.S., there is every reason to believe that al-Qaeda and other such nefarious types will utilize it as a fluid passageway into the states. A large smuggling ring specializing in bringing Middle Easterners across the southern border was broken up in the late 1990s, which suggests the prospect is all too real.

As if government sponsored maps of the U.S. border and armed incursions between suspected Mexican military and U.S. border patrols weren’t enough of a breach of our homeland security, last month a 2,400 yard tunnel was discovered running from Tijuana, Mexico to Otay Mesa, California. Over two tons of marijuana was discovered in the Tijuana warehouse where the tunnel began.

U.S. Immigration and Customs Enforcement officials were quick to admit the national security implications of such a tunnel: “Whether they are designed to smuggle drugs, people, weapons or other contraband, these tunnels pose a threat to our nation’s security.”

The threat posed by such tunnels, 21 of which have been discovered since Sept. 11 when investigations and enforcement were augmented in the region, grows well-beyond the socio-economic burden created by illegal immigrants, drug-trafficking and other contraband. This is a national security disaster.

Notoriously unscrupulous and corrupt Mexican drug-traffickers could easily be paid for their "tunnel services" on a "don’t ask, don’t tell" agreement by such unsavory (and wealthy) characters as al-Qaeda terrorists. While trafficking terrorist members through such sophisticated tunnels would be bad enough -- imagine if these terrorists smuggled in a nuclear weapon or massive bomb components?

If such a scenario were ever to occur, we could not blame naiveté. The facts are startlingly clear: borders are the United States' Achilles heel. If we don’t seriously commit to shoring them up, a worst-case scenario will become reality.

Olivia Albrecht is the John Tower National Security Fellow with the Center for Security Policy in Washington, D.C. Ms. Albrecht researches international relations and national security issues, with a focus on the ‘Islamofascist’ phenomenon. Albrecht previously worked for the Pentagon (Non-Proliferation Policy) and with the Heritage Foundation, and is a graduate of Princeton University with a degree in Philosophy.