Narco-subs, aka drug subs, aka Bigfoot submarines, are custom-made submarines used by cartels in Central and South America to smuggle drugs (usually cocaine) from Colombia to Mexico and/or the United States. For more than twenty years the U.S. Coast Guard has been on the watch for these kinds of smuggling techniques. The first time they found one, they dubbed it Bigfoot, because before they actually found one, they had only heard rumors of their existence.
Sometimes the subs are self-propelled, sometimes, they're dragged by a surface ship. Sometimes the subs are fully submersible, sometimes, they aren't. This all depends on the level of expertise in their creation. The subs can typically carry tons of illegal cargo. Before subs, fishing vessels and speed boats were the main enemy in the war on shipping, but they can be seen on radar and the Coast Guard developed a special kind of helicopter to catch the fast boats. Much of cocaine moved that way was intercepted.
In July 2015, the Coast Guard caught a semi-submersible running just below the waterline.
In the video, the Coast Guard Cutter Stratton crew seizes bales of coke from a self-propelled semi-submersible submarine caught 200 miles off the coast of Mexico, July 19, 2015. They captured more than six tons of drugs from the 40-foot ship, worth an estimated $181 million. Another two tons of cocaine sank with the homemade sub.