The hidden shipyards of the Colombian jungles

In just the last two years submarines have become the source of over a third of the cocaine entering the United States.

MEXICO CITY -- When anti-narcotics agents first heard that drug cartels were building an armada of submarines to transport cocaine, they thought it was a joke.

Now U.S. law enforcement officials say that more than a third of the cocaine smuggled into the United States from Colombia travels in submersibles.

An experimental oddity just two years ago, these strange semi-submarines are the cutting edge of drug trafficking today. They ferry hundreds of tons of cocaine for powerful Mexican cartels that are taking over the Pacific Ocean route for most northbound shipments, according to the Colombian navy.

The sub-builders are even trying to develop a remote-controlled model, officials say.

"That means no crew. That means just cocaine, or whatever, inside the boat," said Michael Braun, a former chief of operations at the U.S. Drug Enforcement Administration.

The subs are powered by ordinary diesel engines and built of simple fiberglass in clandestine shipyards in the Colombian jungle. U.S. officials expect 70 or more to be launched this year with a potential cargo capacity of 380 tons of cocaine, worth billions of dollars in the United States.