José Hernandez is driving down the dirt road on the California side of the U.S.-Mexican border, with his head hanging out the window, staring at the road ahead and to the side of the truck. When you've spent more than 15 years hunting humans, you get pretty good at it.

"You're looking for overturned rocks, twigs that are broken, leaves that have dew missing, things of that nature," he says.

Hernandez has been a U.S. Border Patrol agent in Arizona and California since 2001. It takes only about eight minutes to spot something.

"You can see footprints starting on the side over there and walking down the middle of the road."

Border Patrol agents have names for different types of footprints, with this one known as a 'Mr. Peanut.' (Kim Brunhuber/CBC)

Border agents see the same types of footprints so often they've given them nicknames. The person who crossed this road left an imprint bearing the waffle pattern of a peanut shell. So this footprint, Hernandez says, is a "Mr. Peanut."

He points to hills in the distance.

"Typically from the time they start their walking to the time that they're picked up in this environment here, they'll be out here from three to five days before they'll be picked up somewhere."

According to the Department of Homeland Security, Border Patrol agents in 2015 apprehended about 80 per cent of those who tried to cross the Mexico-U.S. border.

But a leaked Homeland Security report using a different method of counting found they only catch slightly more than half: 54 per cent.

A hat was left on a border fence near Tecate, Calif. (Kim Brunhuber/CBC)

When you see the border, you can understand why.

Hernandez takes me to a spot across from Tecate, Mexico. It's like a museum of border fencing. Three types are on display within about 100 metres.

Primary fencing includes this 'tank trap,' but there are also large sections of the border where pedestrians can simply walk across unimpeded. (Kim Brunhuber/CBC)

The first, most primitive type of barrier, called a "tank trap," is a series of steel Xs that stops only cars, not pedestrians.

Then there's old fencing made of Vietnam War-era corrugated steel airplane landing mats, with a number on each panel.

"We have the numbers on there so that if something happens, if we see people running across the fence or if we're getting assaulted, we can identify it by the number on the fence," Hernandez says.

'Gopher hole'

The problem with the old fencing is it's short — only about two metres tall.

"They typically use makeshift ladders, and they hook them up on top and then slide down the other side," Hernandez says.

Or they'll just go underneath.

Hernandez says they can dig a "gopher hole" in about a minute. The newer fencing called PV1 is taller and extends more than a metre underground.

Older fencing made of landing pads stands next to newer fencing that extends higher and into the ground. (Kim Brunhuber/CBC)

But this spot illustrates another problem. Between the start of the old fence and the new section of PV1 is a space about five metres wide. There's nothing stopping anyone from walking between the sections.

"You can see the trail come straight down through there," Hernandez says.

Agents watch that small area, he says, but it becomes more challenging when the gaps are several kilometres wide.

Asked to show the biggest section of fenceless border in the San Diego sector, Hernandez drives to a hilltop in the Otay Mountains.

Hernandez looks into Mexico and the land that extends for several kilometres without fencing. (Kim Brunhuber/CBC)

"We're looking right into Mexico right here," he says, pointing to a line of trees in the distance. "Essentially this tree line right around here is the border itself."

From here, you can't see a fence except in the distant mountains, where it snakes up the hills like a purple steel Great Wall of China.

"They'll follow the tree line all the way out," Hernandez says. "They go right up that canyon and try to make it through there."

Why no fence?

In the San Diego sector, there are about 22 kilometres where you can just walk into the U.S. Why is there no fence?

"Really couldn't tell you," Hernandez says.

He gets back in the truck and his radio crackles. Agents have spotted a small group in the \hills.

"One appears to be dehydrating, they're arresting him up in some shade," Hernandez says. "Another one tried to assault another agent."

The only actual wall at the border is a brick section less than 100 metres long near Tijuana. (Kim Brunhuber/CBC)

He says many who are caught are good folks who just want a better life, headed to Washington state to "pick apples" or to Idaho to "pick potatoes."

But according federal statistics, about 60 per cent have criminal records.

"I've also apprehended many who are felons or wanted criminals, murderers, rapists, pedophiles," Hernandez says.

At Mile 250 in the San Diego sector, there's a fenceless section known for illegal crossings. (Kim Brunhuber/CBC)

Hernandez doesn't want to talk politics. But the two unions that represent U.S. Border Patrol and customs enforcement workers endorsed Republican nominee Donald Trump for president.

The idea of building a bigger, better wall resonates and there is proof of sorts. In the last 10 years, the U.S. has spent more than $132 billion on border security. In that time, according to the Department of Homeland Security, the number of people successfully crossing the border illegally from Mexico has dropped 90 per cent, from 1.7 million to about 170,000.

Other factors like the improving Mexican economy may have contributed to the decline in crossings. And there isn't always a direct correlation between the presence of a fence and illegal crossings.

Border Patrol drags the dirt roads so footprints show up more easily. (Kim Brunhuber/CBC)

Hernandez drives to the "250 monument," the obelisks that marks the 250th mile of border with Mexico. Here, right next to the monument, the fence simply stops.

"This is an area where we do have a lot of activity coming through here because there isn't a fence," Hernandez says.

But he admits the lack of fencing may not be the sole reason for all the crossings.

"Even areas to the west of here where there's fences in place, we still have a lot of activity," he says.

And despite new fences and technology, the would-be illegal immigrants are still coming.

At the fence between Tijuana and San Diego, repairs have been made after it was cut time and time again. (Kim Brunhuber/CBC)

A truck rumbles behind us. It's soon apparent what's making the racket: the Border Patrol truck is dragging four tires over the gravel road, raising a cloud of grey dust.

"This is what we call dragging the road," Hernandez says. "It erases anything that was there before, so if anything comes across it's more easily detected."

Getting creative

In response, those who are crossing the border get creative.

"They'll put some kind of burlap or carpet on the bottom of their shoes to make it more difficult for us to catch them," he says. One man even used a leaf-blower to erase evidence of his passage.

To see the lengths people will go to, we travel to the border with Tijuana, traditionally one of the most popular spots for illegal crossings.

Here, they've erected at least two and sometimes three layers of fencing. The newest type stretches more than five metres off the ground and is topped by barbed wire.

Hernandez gets out of the truck and points to cuts in the fence. Some are circular, others are shaped like birds' feet.

An innovation being tested has razor wire at the top and the bottom of the fence to discourage climbing and cutting. (Kim Brunhuber/CBC)

"Once they make the cut, they wait to see if we respond, and if nobody responds then [they'll] send a group of people through."

Border Patrol's response? An even newer type of fence with coils of razor wire at the base.

Cameras also do surveillance of the border. (Kim Brunhuber/CBC)

But the truth is, he says, no fence will stop everyone.

"Any time there's a vulnerability that we have along the border, it will be taken advantage of."

His radio crackles again.

"One of the camera operators spotted someone who jumped the primary fence and is trying to make their way across."

Agents report a person crawling slowly through sewer water.

"People want to come across," Hernandez says, and "they're going to find some way to do it."