While Mexico officials and incoming President Donald Trump feud over a border wall, cartels are finding plenty of opportunities to smuggle drugs underground.

A plan by the United States and Mexico to shut down tunnels that the cartels used along borders for smuggling is failing as criminals have found ways to carve them out again, according to the Los Angeles Times.

The reason is because while the United States sealed the entire portion of tunnels on its side of the border, Mexico sealed only the openings on its side, citing a lack of finances.

Proponents of strict border security say that more must be done, by Mexico and a U.S.-Mexico joint effort, to beat back the cartels.

“It would be nice to have full cooperation from the Mexican government,” said Ira Mehlman, spokesman for Federation for American Immigration Reform, or FAIR, which pushes for tighter immigration enforcement.  “If they can’t completely fill the tunnels, then maybe they could adequately police them.”

“Obviously there’s no guarantee that nothing is going to get across,” Mehlman said to FoxNews.com. “But you do your best to make sure it’s as difficult as possible for them, you raise your stakes, you raise their cost” of rebuilding the tunnels and smuggling.

On the U.S. side, drug tunnels have been filled with concrete since 2007, after the Los Angeles Times reported that they were being left unfilled because of budget constraints within Customs and Border Protection.

Since 2007, it has cost Customs and Border Protection $8.7 million to fill drug tunnels, according to a 2016 report by the Department of Homeland Security.

An estimated 20 large tunnels, constructed before and after 2007, are largely intact on the Mexican side, officials told the newspaper.

If the U.S. government had tighter control of illegal immigration, some experts say, it would free up more resources to focus on such things as cartel operations along the border and national security.

“They need to cut off magnets that draw illegal economic migrants,” Mehlman said, “so that we can focus our resources on stopping people who pose a real threat to the country.

Jessica Vaughan, director of policy studies for the Center for Immigration Studies, which also favors strict immigration enforcement, agreed.

Vaughan says the U.S. should attack one of the cartel’s biggest source of finances – human trafficking.

“As long as people think they could reach the United States, they keep paying the cartels to do it,” she said to FoxNews.com, adding that stopping such border enforcement strategies as “catch and release” would be a step toward chipping away at the demand by would-be immigrants for smugglers.

“Our lax immigration policies, our sanctuary policies, and the inability to make progress on drug abuse problems in the U.S. are what have enriched the cartels to begin with,” Vaughan said. “They’re making so much money smuggling through the border.”

The U.S. government focused efforts on filling up large tunnels leading up to the border after U.S. Sen. Dianne Feinstein (D-Calif.) called them “national security risk,” the Times noted.

Even then, however, some U.S. authorities predicted that traffickers would try to make the tunnels usable again, and suggested that the U.S. government pay Mexico’s filling of the tunnels – something that hasn’t happened.

Mexican authorities say they lack the money to completely fill the tunnels, some of which are outfitted with ventilation and rail systems to whisk contraband hundreds of yards under the border. Only the tunnel openings are sealed south of the border.

That allows traffickers to simply dig a new entry point to access the largely intact subterranean passageways leading to the U.S.

A smugglers' tunnel that had been shut down but left unfilled on the Mexican side was found to be back in operation in December, the Times reported Sunday. Traffickers have reactivated or tried to reactivate at least four other tunnels in recent years, most recently last month near Tijuana's airport.

"The biggest threat is that it's a huge open invitation for drug traffickers, and it's definitely going to be taken advantage of," said Michael Unzueta, a former special agent in charge of Immigration and Customs Enforcement in San Diego, to the Times.

Border and drug war analyst Sylvia Longmire said to FoxNews.com: “As long as there’s a demand for drugs in this country,” cartels will get creative about how to get around border walls and impediments to tunnels.

The Associated Press contributed to this report.