Around 12 million people have entered the US from Mexico over the past four decades, both by legal and illegal methods. It’s not clear that how many successfully crossed the border illegally.

Customs & Border Patrol tries to estimate the accurate number based on surveillance footage, evidence of movement (e.g., overturned rocks, footprints, litter) and reports from local residents. Nearly half of all border apprehensions occur near the southernmost tip of Texas as Arizona’s southern border is fenced off that has reduced crossings there, but led to increased crossings in Texas.

According to a recent survey by the Pew Hispanic Center, an arm of the Pew Research Center, tougher border and the weakened US economy are among the factors that have contributed to the sudden drop.