Each year, more than 30 million people flow between the US and Mexico through the San Ysidro Port of Entry, the busiest land-border crossing in the world. Situated between San Diego and Tijuana, at one time the area around San Ysidro was a prime spot to cross illegally into the US. But in 1994, Operation Gatekeeper expanded the border wall and increased the number of checkpoints. With the more recent addition of unmanned drone patrols along the border, Tijuana has become one of the most fortified border points in the Americas. Border crossers have been forced to turn to alternative sites of crossing, such as the Sonoran Desert, where hundreds of people die each year.
About 40 percent of Mexican immigrants deported from the US are sent back through Tijuana. Many of the deported border crossers have established a makeshift shantytown inside a dry, concrete riverbed where the Tijuana River once flowed—called El Bordo.