They are stealth, submersible and smuggling tons of drugs.
So-called "Narco subs" are high-tech homemade vessels being used by drug cartels to move illegal drugs along the coastline of Central America.
U.S. Coast Guard crews based in San Diego say while highly unlikely they’ll be spotted this far north, they are still on the watch for this type of drug smuggling.
The Coast Guard captured video while aboard a narco sub in 2008 off the coast of Guatemala.
The vessel was carrying 15,000 pounds of cocaine and was made mostly from fiberglass with a powerful diesel engine.
It was just one of the many self-propelled semi-submersibles the Coast Guard has intersepted in the Pacific Ocean in the past 20 years.
The subs can cost upwards of $2 million to build and they are often used by drug cartels to move tons of cocaine up the coastline and into places like southern Mexico and Central America.
Coast Guard officials say they can be effective because they are low on the water allowing them to more easily avoid detection.
Lt. Commander Matthew Jones says he has never seen a Narco sub as far north as San Diego.
"The main types of vessels we've seen doing the smuggling are the pangas.. or just normal recreational vessels," said Jones
But according to the Coast Guard, some of the subs are capable of traveling from Guatemala to San Diego without having to stop and refuel.
Although coast guards officials say it is highly unlikely any Narco sub will ever be used as far north as San Diego, it is possible.
That's why the Coast Guard is always on watch along with other law enforcement agencies like ICE.