While the Trump Administration has made building a wall a top priority in its quest to secure the border, Fox News has discovered a major lapse in security that’s currently being exploited by Mexican cartels.

Authorities continue to find underground tunnels, both primitive and sophisticated, burrowed under the border fence that splits San Diego and Tijuana. When they do, American law enforcement fills the entire U.S. side of the tunnel with concrete.

“It's done to every tunnel in order to deter anyone from ever using it again,” said Juan Munoz, Homeland Security investigations special agent.

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But on the other side of the border, Mexican authorities say filling their end is too expensive. Instead, they leave discovered tunnels open underground, allowing the cartels to get back inside.

In the back of an empty Tijuana warehouse, Fox News found a tunnel covered by a steel slab. Underneath, though, it's still hollow. That allows the cartels to reuse the drug passage up to the U.S. border where they can tunnel around the concrete barriers. That saves the criminal organizations time and money.

“There's been a least seven documented incidents where that has actually happened,” said Lance Lenoir, captain of the U.S. Customs and Border Patrol “tunnel rats” team.

It's a major frustration for American authorities who, according to this report, spend millions to discover and then shut tunnels down.

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In Mexico, Fox News was able to walk up to one tunnel entrance after another, finding that security is lax or non-existent. Some tunnels are left unguarded. Others are watched over by guards who claim they’re being paid by the government.

“How many tunnels are in this neighborhood?” Fox News Correspondent Will Carr asked one guard in Tijuana.

“Six,” he responded.

“How concerned are you for your safety that the cartel will try to get back in this tunnel?” Carr asked.

“If the cartel comes back I'm not going to fight them,” the guard said through an interpreter. “They're just going to come right in,” he added, waving his hands in an inviting type motion.

Experts say that's the typical response by officials in Mexico.

“The only thing that they do is to send a couple of militaries or federal police to guard the tunnel for probably a couple of months and then they forget about the tunnel,” said Victor Clark-Alfaro, a tunnel expert who lives in Mexico and also teaches at San Diego

State University.

American authorities tell Fox News the best ways they're combating tunnels on this side of the border is by using technology and information they get from informants and the surrounding community.