Nogales, Arizona (CNN) -- As night falls, U.S border patrol agents know smugglers will use the dark to sneak into the country with illegal immigrants and drugs -- both above and below ground.
Under the Arizona border city of Nogales is a network of tunnels -- some built as part of the city's flood defenses, others built by the smugglers.
U.S. Border Patrol agent Michael Camdron says: "We're usually trying to see some sign that someone's been coming through, pushing bundles of marijuana or something like that."
In the U.S. illegal immigration is a hot political potato and in Nogales it's a part of everyday life as immigrants use the area as a crossing point.
The Grand Tunnel was built right underneath the border crossing at Nogales to relieve both sides of the border during the monsoon season.
Agents get into the tunnel by lifting a heavy grate and climbing down a metal ladder. Once inside the tunnel, only flashlights separate them from complete darkness as they inspect the mile-long structure.
Agent Ariel Medeles, who is in joint charge with Camdron of inspecting the tunnel, says they have various techniques to find out whether anybody has tampered with the tunnel's security.
"You can set up a marker, an empty can or something standing up. That way you know you set it there. If anybody comes through, they'll knock it down or push it out of the way," says Medeles.
There are two other professionally built tunnels in the Nogales area. The Morley Tunnel is also a mile long and it presents the same challenges for Border Patrol agents. Then there's a tunnel on the Mexican side of the border that runs east-west.
In 2009, agents discovered 20 makeshift tunnels connecting to the bigger ones or isolated that were being used by smugglers. The number of tunnels decreased to seven last year. So far in 2011, three have been found.
Patrolling this tunnel is not for the faint of heart. While searching for signs of human activity they come across black widow spiders and other insects, and entire colonies of cockroaches.
At the end of the tunnel lies a gigantic metal gate that separates the Mexican from the U.S. side. A faint light bulb partially illuminates the area around the gate. Trash and debris are everywhere.
Medeles said: "A lot of times they'll tunnel around the gates and they'll come out on the north side and they'll bring people or contraband across, drugs or I have seen that they'll push the gate, five or six guys, and they'll run across too."
Camdron added: "I've personally seized groups of 30 individuals with 50-pound bundles [of drugs]."
Before the gate was installed, it was not uncommon for agents to find dead bodies in the tunnel. Medeles said: "There was a group down here and then they got caught in the flash flood and hours later they were looking for a body north of the openings."
Above ground Border Patrol Agent David Jimarez is parked a few hundred feet from the border fence when sudden chatter on his two-way radio catches his attention.
"We've got two approaching the ravine; make that three," says another agent patrolling the same area of Nogales.
The agents know they only have minutes to find the suspects otherwise they will disappear in the darkness.
Rudy Garcia, an agent who patrols this section of Nogales on a bicycle, has made visual contact with a suspect. "He jumped the fence and he didn't stop. He just kept running," Garcia says.
Half a dozen agents start searching for the man, but it's not an easy task. Vegetation and shrubbery cover most of the hilly area.
The Border Patrol has installed floodlights, but the light only goes so far and agents have to use their flashlights to look in and around the multiple potential hiding places.
Fifteen minutes later, Garcia finds the man hiding underneath a platform in a backyard. "You have to search every little crevice and every crack because, you know, they can hide anywhere," says Garcia.
The man is lying flat on his belly and seems a little nervous, but otherwise in good shape. He says he's 38 and comes from the state of Oaxaca in southern Mexico. He quietly mumbles a few words about what motivated him to make the trek north.
"My family and poverty in Mexico... I want [something] better. I just want a job to support my family," he says.
Not far away, seven more would-be immigrants have been arrested including 32-year-old Cain Meza Aguirre.
He says he comes from the Mexican state of Nayarit; he has a wife, two boys, a girl and a baby on the way. He was after a job to support his family, but now he's in detention.
"Back to Mexico with my family and never come back. That's what I want," he says before being put in a transport vehicle that will be taken to a station for processing.
Nogales has been on the front line of the fight against smuggling organizations for years. In 2000, agents arrested 616,000 who illegally crossed the border, according to statistics from the U.S. Border Patrol. That number dropped to 439,000 in 2005.
Last year the number of arrests was 212,000, but the dynamics have changed.
Jimarez says larger and more sophisticated smuggling organizations are part of the new challenges.
"These smuggling organizations like to operate from dusk till dawn. They like to operate under the cover of darkness because they think they're not going to be seen," Jimarez says.
"The whole smuggling organization has changed. It's not your mom-and-pop-shop anymore. Now everything is organized crime," he added.
State-of-the-art technology in a control room located near the border and equipped with infrared technology gives agents eyes in the dark.
Agents closely watch for movement along the U.S.-Mexico and report anything unusual to colleagues positioned along the border in SUVs, or on ATVs and bicycles. They also have the option to respond by helicopter.
There are also surveillance towers and an agent force that has more than doubled in the last 10 years.
There were roughly 1,500 agents in the year 2000 for this area -- the Tucson sector which covers 262 linear miles along most of the Arizona border with Mexico. Now the regional force has more than 3,400 people.
But some of the old realities remain. At the border there are rocks everywhere and they are a useful weapon to throw at the border agents, who say they're attacked every day. SUVs show the signs of damage and agents riding bicycles are especially at risk.
Agent Ariel Medeles says he has seen agents get injured by projectiles flying from the other side of the fence. "They're not your average-size rocks," says Medeles.
Agents recently confiscated 1,100 rounds of ammunition going south into Mexico, presumably to be used by a drug cartel. In a separate incident, a manhole located about 100 yards from the border had to be welded shut because it was being used to smuggle marijuana, according to Agent Michael Damron, who also patrols the fence area on bicycle.
And this night there's a new arrest. She's an 18-year-old girl from the Mexican state of Veracruz.
For her, it's the end of a 1,200-mile trip in search of a dream. For the agents, she's one more captured illegal immigrant. Generally, they say, they make more than 300 arrests a night.