Authorities constantly search for contraband at the San Ysidro-Tijuana border. Customs and Border Protection officers look for anything suspicious.
Every day more than 80,000 people, on average, pass through the San Ysidro port of entry.
"The scope of this port is huge. We're the biggest land border crossing in the world," said Frank Jaramillo, U.S. Customs and Border Protection Area port director.
Since the 9/11 terrorist attacks, many people crossing into the U.S. have noticed changes.
The closer scrutiny includes having proper documentation, such as a passport.
"Our mission is first and foremost to prevent terrorist and terrorist weapons from entering this country," said Jaramillo.
Authorities say they have better technology since 9/11, such as radiation-detection equipment, and X-ray machines that can discover drugs and humans hiding in cars.
Officers tore open one vehicle after an imaging device tipped them off. Hidden in the firewall were 17 bags, 30 pounds of methamphetamine. The 44-year-old Mexican driver is arrested.
At the same time, in the passenger inspection area a woman is taken into custody. Officials say she was posing as someone else to try to enter the U.S. illegally.
"We average anywhere from 100 to 118 apprehensions a day here," said Jaramillo.
"There is a lot of brush, many locations where someone could hide," said U.S. Border Patrol Agent Kurtis Woods.
Over the past 10 years, the number of Border Patrol agents in the U.S. has doubled to more than 20,000. There are also physical changes.
The fence here separating the U.S. and Mexico went up in the early 1990s. In 2009, for extra security, a secondary fence was added.
"The camera towers, we've added. So all of this combined has given our agents more time to respond to any threat that comes across that border," said Woods.
Officials say the number of people caught crossing illegally has dropped by about 65 percent since 2001.
On this day agents, spot 10 people hiding, waiting to hop the fence into the U.S.
Officials admit criminals are getting creative.
"They have not stopped coming across. They've moved to other locations," said Woods.
Authorities have discovered 40 tunnels connecting Mexico with the U.S., most dug after 9/11. Hundreds of people have also been caught travelling by water.
Some question that if regular people can still sneak into the U.S illegally, what's to stop highly trained terrorists?
The port director insists we are safer due to better sharing of information with other agencies, tighter security, along with a change in thinking.
"We have to be nimble in an organization so that we can change to meet the changing threats," said Jaramillo.