As the border wall is erected between the United States and Mexico, the drug trade does not stop underground.
Patrols from both countries find tunnel after tunnel, drilled into the 80-year-old drainage system that connects the two countries 15 feet below ground.
On Saturday they located the last one, excavated by hand near the Mexican border of Nogales.
Agents located the two-foot by two-foot tunnel exit about a half mile west of the DeConcini port of entry and about three feet north of the border.
The Mexican National Guard located its entrance on the floor of the existing Grand Avenue Drainage System in Mexico.
This is the 127th tunnel discovered in the Tucson sector since 1990 and the first discovered this fiscal year.
The narrow passage stretched about 10 feet and had no shoring, ventilation or lighting,
according to authorities.
“Nogales is the capital of the cross-border tunnels between Mexico and the United States,” Ricardo Santana Velázquez, Mexico's consul general in Nogales, Arizona, tells The Washington Post.
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Officials say that cooperation between the United States Border Patrol and the Government of Mexico plays a vital role in border security.
Cocaine often travels north through the sewers: sometimes floating in bags on a river of sewage, sometimes dragging with the merchandise through mud and excrement until it reaches US soil.
A drug trafficking tunnel from Mexico discovered by the U.S. Border Patrol in Nogales, Arizona, on November 22, 2011. Reuters
Seven miles of drainage and sewer lines are part of the Mexican side, guarded in the front line by six national guards with flashlights. A daily challenge since the traffickers discovered the Nogales drainage system and began drilling. Authorities do not know how much of the drug is leaking, but they estimate that a hand-dug tunnel could be worth tens of millions of dollars to drug cartels, according to The Washington Post.
The Mexican National Guard team has found tunnels that led to places like a weedy patch of grass or even a cemetery, where traffickers had removed a corpse to enter the pipeline through the grave.
The drainage system was built in the 1930s between the two countries and connects the twin cities of Nogales, Mexico and Nogales, Arizona.
When it was completed in the early 1940s, no one knew the possible consequences.
More than half of the tunnels found under the US-Mexico border since 1990 were discovered there.
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Guarding the 2,000-mile (3,218-kilometer) land border between the United States and Mexico is difficult enough, but patrolling underground in an aging drainage system is nearly impossible -
the drainage system is made up of a combination of about 16 miles of pipe, tunnel and open channel
that crosses the border through which between 11 and 15 million gallons of water per day pass from Mexico to the US, where they are treated.
"There's a reason why they call Nogales the capital of the border tunnel,"
John Mennell, a spokesman for US Customs and Border Protection, told The Washington Post.
"But it also underscores the importance of the wall. These traffickers have been driven underground because the easier routes are no longer available," he added.
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The US surveillance team has a tunnel simulator to train its employees and uses state-of-the-art technology to patrol, such as devices that measure levels of oxygen or noxious gases.
"Americans have it all.
We don't even have helmets, "lamented a member of the Mexican unit to The Washington Post.
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On the Mexican side, the roof of the tunnel system appears to be weakening
and local officials have expressed concern about the possibility of collapse.
"Every time someone pierces the side of the infrastructure, it weakens it even more," recalled Luis Ramírez, advisor to the Port Authority of the Greater Nogales Santa Cruz County.
“Law enforcement agencies on both sides often patrol the tunnels together.
We understand that this criminal activity affects border security, said Santana, the Mexican consul general.
An underground tunnel in Arizona, on the U.S.-Mexico border, is plugged with concrete on May 22, 2014. Reuters
In several cases, the Mexican patrol has found men in the sewer who appeared to be digging.
But because they were technically still on Mexican soil and not in possession of drugs, no charges were filed.
"Cross-border tunnels are the parallel and underground reality of the illicit trafficking of weapons, money, drugs and people, which can no longer be ignored," Santana recalled.
With information from The Washington Post.