SAN DIEGO -- The border fence gets no respect.
Drug traffickers have smashed trucks into the barrier, trying to break through.
Coyotes routinely use axes and battery-powered saws to slice the steel mesh.
To borrow Carl Sandburg's phrase, though, the sneakiest foe comes on little cat feet. "Fog," said U.S. Border Patrol Officer Wendi Lee. "Fog is the big issue."
To reduce the northward flow of drugs and undocumented migrants, should the United States wall off its border with Mexico? GOP presidential candidate Donald Trump is this tactic's most recent advocate. The idea dates to at least 2005, when Duncan Hunter -- then the congressman representing San Diego's East County -- urged a fence along the entire 1,954-mile boundary.
Not everyone agrees. Critics say walls are simplistic responses to the complexities of immigration. (A Texas congressman called this "a 14th-century solution to a 21st-century problem.") Defenders counter that every nation is entitled to guard its sovereignty. Other countries, including Israel and Saudi Arabia, have border fences.
Also on that list: The United States, where fences cover about 670 miles of the border. San Diego County, which abuts Mexico for 60 miles, has 46 miles of fence and a double layer covering 13 of those miles.
Keeping California's border fence intact is a $9 million-a-year job requiring surveillance cameras, underground sensors, stadium lights and roving bands of welders. In the stretch between the San Ysidro and Otay Mesa border crossings, human-sized holes are punching through the fence about 550 times a year.
On mornings when the coastal air drips with fog, limiting the Border Patrol's visibility, this daily occurrence seems to happen hourly.
"It could happen once, twice a day," said agent R.C. Martinez. "When it's foggy, it could be more -- you'll have three, four, five compromises a day. When it gets foggy here, it gets really foggy."
When a pro only needs 30 seconds to cut the fence, clouds of fog act like cloaks of invisibility.
Martinez, 52, was raised in San Diego. He remembers the years before the fence, when people flooded the open "soccer field," waiting for nightfall to mask their northbound passage.
Then came Operation Gatekeeper, bulking up the Border Patrol's presence -- in the San Diego sector, staffing has tripled. Then the primary fence, about 8 feet tall. A second fence, almost twice as tall. Networks of cameras perched on tall poles and electronic sensors buried in the earth.
Migration patterns shifted east, into mountains and across desert. Still, thousands of determined souls leapt the primary fence and raced to the secondary.
For a few years, some used ladders to reach the top. These same people often leapt to the ground -- and landed in local hospitals with broken bones. For greater security and fewer medical expenses, the secondary fence is now capped with concertina wire.
"Everything is a layered approach," Martinez said. "Primary fence, secondary fence, concertina wire, camera."
In this chess game, each move prompts a counter-move. Standing in the no-man's-land north of Tijuana's Colonia Libertad, agent Jose Hernandez noted that the hilly terrain is stripped of structures on the U.S. side. The Mexican side is densely built up, with everything from shacks to mansions.
"They can see everything we do," Hernandez said.
"They" are the coyotes who charge migrants $3,000 to $5,000 per head to slip through the fences. Assembling customers in the colonia, they await an opening -- fog, twilight, a few minutes between Border Patrol trucks -- then two coyotes hop the primary fence and dash to the secondary.
They work quickly, methodically. One man swings an ax, cracking the metal mesh near the ground. The second uses a battery-powered saw to rip open an upside down U, about 2 feet high and 2 feet wide.
"From the time they punch a hole in the fence," Martinez said, "it takes about 30, 40 seconds to make the cut."
Their clients rush north, slipping through the opening. The coyotes, though, grab their tools and hurry south, their job done.
"They want to get in, make their cut and peel out back to Mexico," Martinez said. "Cut and run."
That's when the maintenance crew goes to work.
Between San Ysidro and Otay Mesa, the tall secondary fence's lower sections resemble a metal quilt. Squares of fresh mesh are welded or bolted across holes; thousands of dark curved lines show where welders have closed gaps.
On a recent morning, Nathan MacCallum stood by the fence, dressed in heavy boots, jeans, a jacket and a helmet with a dark glass filter shielding his eyes. When a 220-volt wire was linked to the fence, MacCallum touched a foot-long stick of steel to the surface. An electrical circuit was completed, and fence and stick began melting and melding.
Martinez stood to the side, holding a fire extinguisher. Another agent, Tom Maloney, also stood watch.
"You have to have at least two agents," Lee said. "One to keep an eye on things, the other welding."
Focusing on the fence leaves you vulnerable, as Lee knows from experience. A few years ago, she was welding part of the fence alone. From a nearby hillside, another agent radioed a warning -- behind Lee's back, people carrying rocks and clubs were closing in on her.
As the agent hustled to Lee's side, the band dispersed. While many Border Patrol officials in this sector have tales of being "rocked," targeted with stones, bottles or other projectiles, assaults on agents peaked in 2007 (137) and have been steadily declining since 2012. Last year, there were 60.
While drug traffickers have tried to breach the fence, agents say most who attempt this route smuggle people instead of narcotics. Conversely, tunnels beneath the fence are primarily used to move drugs, not humans.
The reason? The drug cartels who invest thousands of dollars in elaborate tunnels avoid unnecessary risks.
"People talk," Hernandez said. "Drugs don't."
Border Patrol agents have heard no details about Trump's proposed border-to-border wall, nor are they willing to speculate.
"I wouldn't presume to speak for Mr. Trump," Hernandez said.
Still, it's known that previous administrations -- even the most pro-fence -- have left gaps in this barrier. Some portions of the border are so rugged or remote, nature itself acts as a deterrent.
"I'm a logical person," Martinez said. "I don't think we can make a fence complete."
It's also expensive. In 2009, the federal government estimated that adding 3.5 miles of fence to a rugged part of San Diego County would cost $58 million, or just less than $1 million per 100 yards. In 2007, the Congressional Research Office estimated the cost of building 700 miles of fence, and then maintaining it for 25 years: $49 billion.
Martinez argued that the border is more secure today than it has been in years. In 2015, apprehensions in the San Diego sector were the lowest (26,290) since 1968 (24,116), and a 96 percent decrease since the 1986 peak of 629,656. He credits that to more agents and better technology.
Still, he's not declaring victory.
"If there is a way," he said, "they'll find a way. It's a never-ending battle."