They are known in the U.S. Border Patrol as "tunnel rats" -- agents who go in clandestine passages that have proliferated on the U.S.-Mexico border over the past 20 years to smuggle drugs.
The Associated Press joined the Border Tunnel Entry Team, as it is formally known, inside an incomplete tunnel that was discovered in San Diego in 2009 -- 70 feet deep, 3 feet wide, 2,700 feet long and equipped with a rail system, lighting and ventilation.
Here are some questions and answers about the team's work:
How many tunnels are there?
Authorities discovered 224 border tunnels originating in Mexico from 1990 to March 2016, including 185 that entered the United States, according to the latest U.S. Drug Enforcement Administration annual survey. Many are shallow holes, but some are elaborately constructed with hydraulic lifts, water pumps and rail cars.
The vast majority are in Arizona, where smugglers connect to underground drainage canals in Nogales, and in California, where construction noise generates less attention amid warehouses of an industrial area of San Diego, across from densely packed homes and businesses in Tijuana.
What are they for?
Tunnels are generally used for multi-ton loads of marijuana because the drug's bulk and odor are difficult to conceal for motorists and pedestrians who enter the United States at official border crossings, the preferred method for smuggling methamphetamine and heroin.
In 2015, authorities seized cocaine in connection with two California tunnels, including one that ran underwater from a house in Mexicali, Mexico, to the All-American Canal near the city of Calexico.
The tunnels, which the DEA generally attributes to Mexico's Sinaloa cartel, cost between $1 million and $2 million to build and take months to complete, said Chris Davis, supervisory special agent with U.S. Immigration and Customs Enforcement's Homeland Security Investigations. That investment quickly pays off with profits from smuggling if crews escape detection, he said.
How are tunnels found?
Leads from informants, neighbors and others have been the most trusted technique, but technology plays a part. Lance LeNoir, who leads the Border Patrol's "tunnel rats" team in San Diego, says seismic devices, acoustics and ground-penetrating radar complement human intelligence.
Investigators keep tabs on who owns and rents warehouses in San Diego's Otay Mesa area for suspicious transactions. They also visit businesses to ask them to report telltale signs: construction equipment and piles of dirt, jackhammer sounds, people coming and going at odd hours.
"They'll tunnel anywhere they want to. It's wherever they can get a building on the south side and a building on the north side," LeNoir said. "Location, location, location."
Sometimes agents stumble across "gopher holes" while on patrol.
Once the passages are found, the "tunnel rats" go inside, a dangerous assignment because there's always a chance the walls can collapse. They map and measure the passages and work on filling them with concrete to prevent them from being used again.
What happens after tunnels are discovered?
On the U.S. side, the tunnels have been filled since 2007 to prevent smugglers from burrowing into them. In Mexico, they are sealed but not plugged with concrete.
Mexican authorities say they don't have the money to fill them, a vulnerability that is gaining more public scrutiny. LeNoir says smugglers have tapped into existing tunnels at least seven times in recent years.
"It gets down to funding and political will," he said.
The U.S. Department of Homeland Security says it spent $8.7 million to fill tunnels from 2007 to 2015. Last week, it awarded a $153,000 contract to inject concrete into the U.S. part of a completed tunnel lined with cobblestone. The tunnel was discovered in October. It ended in a San Diego warehouse 1,200 feet north of the border.
What does president Donald Trump want to do?
Trump has made construction of a "great wall" on the 2,000-mile U.S. divide with Mexico a signature issue of his presidency, prompting critics to say that people will go over, under and around it.
Trump, as the Republican nominee, promised during an August speech in Phoenix that he would "find and dislocate tunnels and keep out criminal cartels." His executive order on border security doesn't specifically address tunnels but notes that criminal organizations run sophisticated drug and human smuggling networks on both sides of the border.
After going inside a San Diego tunnel underneath a highly fortified border fence last month, Homeland Security Secretary John Kelly said drug profits were so enormous that smugglers would keep trying to burrow through.
"I would argue that the fact that they're spending huge amounts of money to tunnel underneath the wall tells you that they can't get through it," Kelly said.