The Otay Mountains east of San Diego have for years provided an open corridor for illegal immigrants entering the United States.

But now a unique air unit is closing up this gap, flying in by helicopter every day to catch unsuspecting immigrants.

Part mountain goat, part jackal, the 54 agents of the Border Patrol's Air Mobile Unit trek up and down the mountains to swoop in on immigrants and the groups that smuggle them into America.

National Guard and Border Patrol helicopters drop two-man teams — mostly ex-military members equipped with night scopes, infrared devices and tracking dogs — into remote canyons and steep ridges on every other peak of the mountain range just north of the border. Their shift runs from dusk till dawn.

Click here to see the Air Mobile Unit in action.

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.”

By day, the unit's agents use a device they call the “magic cloak,” a camouflage net that conceals agents waiting to ambush the immigrants. By night, the infrared scopes and night-vision goggles help locate people moving in the darkness.

Agents also use scent dogs to track smugglers; some smuggling rings have put bounties out on the canines.

Border Patrol agents said increased pressure in the area and a better border fence have pushed more illegal immigrants toward these mountain passes near San Diego.

And while it’s a small front in the immigration war, the unit thinks that patrolling every part of the border is vital.

“Quite frankly, I don’t think cost is a question,” McPartland said. “Do we cede this part of the United States to Mexico or some smuggling operation?”