THE CRAFT FIRST surfaced like something out of a science-fiction movie. It was November 2006, and a Coast Guard cutter spotted a strange blur on the ocean 100 miles off Costa Rica. As the cutter approached, what appeared to be three snorkels poking up out of the water became visible. Then something even more surprising was discovered attached to the air pipes: a homemade submarine carrying four men, an AK-47 and three tons of cocaine.

Today, the 49-foot-long vessel bakes on concrete blocks outside the office of Rear Adm. Joseph Nimmich in Key West, Fla. Here, at the Joint Interagency Task Force South, Nimmich commands drug-interdiction efforts in the waters south of the United States. Steely-eyed, gray-haired and dressed in a blue jumpsuit, he showed me the homemade sub one hot February afternoon like a hunter flaunting his catch. “We had rumors and indicators of this for a very long period beforehand,” he told me, which is why they nicknamed it Bigfoot.

This kind of vessel — a self-propelled, semisubmersible made by hand in the jungles of Colombia — is no longer quite so mythic: four were intercepted in January alone. But because of their ability to elude radar systems, these subs are almost impossible to detect; only an estimated 14 percent of them are stopped. And perhaps as many as 70 of them will be made this year, up from 45 or so in 2007, according to a task-force spokesman. Made for as little as $500,000 each and assembled in fewer than 90 days, they are now thought to carry nearly 30 percent of Colombia’s total cocaine exports.

These subs are the most ingenious innovation in the drug trade. But as Joe Biden told Congress last July, that’s not the only reason they pose “a significant danger to the United States.” In late January, a Sri Lankan Army task force found three semisubs being built by Tamil rebels in the jungles of Mullaitivu. “With this discovery the [Tamil] will go down in history as the first terrorist organization to develop underwater weapons,” the Sri Lankan ministry of defense declared.