CALI, COLOMBIA -- It was on a routine patrol that the Colombian coast guard stumbled upon an eerie outpost amid the mangroves: a mini-shipyard where suspected drug traffickers were building submarines.

Perched on a makeshift wooden dry dock late last month were two 55-foot-long fiberglass vessels, one ready for launch, the other about 70% complete. Each was outfitted with a 350-horsepower Cummins diesel engine and enough fuel capacity to reach the coast of Central America or Mexico, hundreds of miles to the north.