SAN MIGUEL, Ariz. - A pickup truck in Mexico pulls up to the 5-foot vehicle barriers that make up part of the multibillion-dollar border fence. A retractable ramp is extended from the truck, forming a bridge up and over the barriers.

Then, a second pickup - this one loaded with a ton of marijuana - rolls over the bridge and into the U.S.

With gadgetry such as custom-built ramps as well as ultralight planes, false doors and good old-fashioned duct tape, smugglers have demonstrated unbounded creativity when it comes to sneaking drugs across the Mexican border. And the U.S. government acknowledges there is only so much it can do to stop the flow.