SAN DIEGO -- Tightening border security has forced somed illegal immigrants underground, border officials report.

A growing number of illegal immigrants are crawling through San Diego's vermin-infested storm drains to avoid the Border Patrol. The 23-mile system of pipes surfaces in hundreds of storm grates and manhole covers in the industrial Otay Mesa area. Muddy immigrants make their way through the narrow pipes, which often carry toxic industrial sludge. They climb out of manholes and storm grates in Otay Mesa and take public transportation, taxis or are picked up in private vehicles to get from there to downtown San Diego, officials said.

Border agents are have begun working with city engineers to map out the drain system. They plan to install motion sensors along the system to alert agents of movement. City crews have also begun locking or welding shut manhole covers in some areas. NBC 7/39 reported.

"This is probably one of the most dangerous ways a smuggler can try to gain entry," Border Patrol spokesman Kurstan Rosberg told NBC 7/39. "It's a scary feeling, I'm sure, to go through those tunnels. It's pitch black. You can't see your hand in front of your face."

Officials said immigrants pay smugglers between $1,500 and $3,000 to be guided through the drain system.

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