Cartels are using these 'narco-submarines' to move tens of thousands of pounds of drugs at a time

As the United States, Mexico, and Colombia intensified their war on drugs throughout the late 1990s and the 2000s, the cartels had to reimagine various ways that they could smuggle cocaine into the US. With billions of dollars in annual revenue at stake, no idea for getting drugs into the US buyers was considered too outlandish - Sinaloa cartel leader Chapo Guzman even pioneered the use of cross-border drug catapults.

But the ultimate in high-risk, high-reward smuggling is the "narco submarine," homemade subs that can bring thousands of pounds of product to the US at once.

According to a US Foreign Military Studies Office (FMSO) report on narco submarines citing Drug Enforcement Administration statistics, 80% of drugs smuggled into the US in 2012 came from maritime routes. And 30% of the drugs that arrived in the US by sea were conducted via narco submarines. Like narco tanks, narco submarines show how cartels have mastered do-it-yourself engineering. Even so, around one in four of the vessels are interdicted. US authorities have captured narco subs with as much as 7.5 tons of cocaine onboard.

There are four broad categories of vessel that fall under the narco submarine label: low-profile vessels, semi-submersibles, submersibles, and towed narco "torpedoes."

These vessels have shown a notable leap in quality since they first debuted over 20 years ago.

The first narco sub detected in 1993 was built from wood and fiberglass, could not submerge, and could only travel at 10 miles per hour. But the FMSO notes that the latest models of subs can mask their heat signature, evade sonar and radar, and use lead siding to help mask their infrared signature, making their detection and capture extremely difficult.

Here are some narco subs that the authorities have captured over the years - evidence of the tenacity and resourcefulness of drug trafficking organizations that have to get their product to the US at any cost.

Low profile vessels (LPVs) are one of the most common narco sub variants. These vessels sit just above the water line. They aren't entirely submerged, but they're still difficult to spot.

Their fiberglass and lead construction also render them difficult to detect through infrared. And because they sit almost below the water radar and sonar can have a difficult time spotting them.

Larger LPVs can carry upwards of 10 tons of drugs at at time.

The majority of narco submarines discovered have been LPVs, perhaps because cartels find them easier to construct and operate than fully submersible vessels.

Semi-submersible narco submarines are similar to LPVs. These vessels can completely lower themselves below the waterline - except for a snorkel-like tube to ensure the crew doesn't suffocate.

Fully submersible narco submarines are a rarity due to the cost and technical difficulties of building a working model.

But a few submersibles have been found over the years, and they're impressive.

The largest was a 100-foot long, GPS-equipped craft that could dive to 30 feet and transport upwards of 200 tons of drugs at a time, according to Colombian authorities.

Narco torpedoes are the least technologically advanced submersible. These empty canisters are designed to be dragged behind a camouflaged ship. In the event of detection, the tow-ship can drop the torpedo which then activates a homing signal for later pick-up.

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