In four years, the Air Mobile Unit has arrested 20,000 undocumented immigrants and turned back about 3,000 more, though they estimate about 4,000 have gotten away.

Where they once caught none, the unit now captures between 30 and 100 immigrants a day.

For decades, smugglers used the rugged Otay Mountains — physically imposing and largely inaccessible — as a hideaway from the Border Patrol. Broiling hot in the summer, freezing in the winter and flush with rattlesnakes in the spring, the mountains provided a reliable if difficult passage that agents, lacking road access, were unable to patrol.

“Before the unit was created, there would be days before anyone would get down there just because of the terrain,” Agent Steve McPartland said. But the unit has changed all that.

“Is it working? Yes,” Agent Pedro Olvera said. “We are probably 80 percent effective.”