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A narco submarine (also called narco sub, drug sub and Bigfoot submarine) is a type of custom-made ocean-going self-propelled semi-submersible (SPSS) vessel built by drug traffickers to smuggle drugs. They are especially known to be used by Colombian drug cartel members to export cocaine from Colombia to Mexico, which is often then transported overland to the United States.
First detected in 1993, they are popularly called submarines, though strictly speaking they are semi-submersibles since they cannot dive; most of the craft is submerged with little more than the cockpit and the exhaust gas pipes above the water. In other words, a narco sub is a surface vessel with a very low freeboard or low profile. Narco submarines are designed specifically to be nearly undetectable visually or by radar, sonar and infrared systems.
http://en.wikipedia.org/wiki/Narco_submarine