Just days after the announcement that the U.S. government was stepping up drug interdiction operations in the Pacific and Caribbean, Secretary of Defense Mark Esper used the imagery of a so-called “narcosub” to show that the effort was paying off. Now, naval warfare authority H.I. Sutton has broken down the four most popular types of narcosubs, which drug cartels and other groups use to ship drugs north from South America.

The Enhanced Counter-Narcotic Operations in the Eastern Pacific and Caribbean includes U.S. Navy destroyers and littoral combat ships, U.S. Coast Guard cutters, and P-8 Poseidon maritime patrol aircraft, E-3 Sentry airborne early warning aircraft, and E-8 reconnaissance aircraft.

The planes, detect ships, and aircraft, set up an electronic tripwire fence while the warships and cutters intercept suspicious craft at sea. On April 13 the Coast Guard announced that the cutter Mohawk had intercepted a fishing boat in the Eastern Pacific loaded with 1,700 pounds of cocaine. The boat was described as having been detected the week before, likely by a radar-equipped aircraft such as the E-8, and the Mohawk moved in to intercept it.

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These subs are typically constructed on the coast of South America and move drugs north to Central America, Mexico, and the Caribbean. The map above, produced by U.S. Southern Command, shows the routes narcosubs typically take, bringing their cargo closer to the U.S.

Read more: Forbes/Popular Mechanics