Scores of trucks pass through Otay Mesa, California, each day, loading and dropping off cargo at various warehouses located just yards from the U.S.-Mexico border.

One warehouse bearing the name "Medi Int Enterprises" -- an alleged storage facility for toilet paper -- seemed to be running business as usual, equipped with a front desk and receptionist. Yet, on Nov. 3, 2010, Immigration and Custom Enforcement agents found they were storing much more than bathroom supplies.

ICE agents intercepted a truck leaving the warehouse and discovered 20,000 pounds of marijuana. An additional 32,000 pounds were found in 10-kilo bricks throughout the warehouse, making it the second largest seizure of marijuana in U.S. history. But, the biggest find was what lurked below.

Agents found 1,800 feet of tunnel running underneath the warehouse to Mexico. Designed with tracks and pulleys, the smuggling tunnel is the most recent to be discovered in a growing number popping up along border states.

Since Sept. 2001, 113 tunnels have been discovered -- a 63 percent increase in just the last two years. ICE agent Tim Durst said they are becoming an increasing threat with the “growing presence of law enforcement above ground.”

With tougher enforcement and new barriers along the U.S.-Mexico border, smugglers have become more creative in their methods, finding alternative routes by sea and land and now underground.

Seventy-one smuggling tunnels have been uncovered in Nogales, Arizona, with San Diego, California, a close second, counting 34 in both Otay Mesa and San Ysidro. The tunnels are "golden goose eggs" and if they're completed and become operational, they are of great value to smugglers, Durst said.

The majority of the tunnels ICE agents have found are unfinished, which makes this find that much more concerning to law enforcement. Durst says the tunnel in Otay Mesa took approximately a year to build, but had been operational for less than a month.

While the tunnels can be sophisticated drug smuggling routes, the engineering behind the creation of the tunnels is not. Would-be smugglers use jackhammers, concrete saws and shovels that can be purchased in retail hardware stores.

Law enforcement isn't empty-handed in their efforts to thwart smugglers from taking over the underground. Durst is part of a tunnel task force, monitoring areas like Otay Mesa and looking for signs of things out of the ordinary.

New technology has aided ICE agents in their battle against smugglers. Ground-penetrating radar detects anomalies in the ground and can find trap doors or exits to potential tunnels. A motorized robot -- controlled like a video game from above ground --serves as a safety precaution. It has a camera on top and navigates through tunnels, allowing law enforcement to scope out the scene before entering.

Joint efforts by ICE and other government organizations including the Department of Defense, universities, and Border Patrol are working to create newer and more effective technology to combat the growing problem of smuggling tunnels.