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 …