A Coast Guard Cutter Stratton boarding team opens the bridge of a self-propelled semi-submersible interdicted in international waters off the coast of Central America, July 19, 2015. The Stratton’s crew recovered more than 6 tons of cocaine from the 40-foot vessel. (Coast Guard photo courtesy of Petty Officer 2nd Class LaNola Stone)

ALAMEDA (CBS SF) — A U.S. Coast Guard crew from Alameda stopped a semi-submersible vessel carrying more than 16,000 pounds of cocaine in the Eastern Pacific Ocean last month — the largest bust of its kind in Coast Guard history.

On July 18, the crew apprehended four suspected smugglers and captured 275 bales of cocaine worth more than $181 million wholesale from the self-propelled semi-submersible (SPSS) vessel, a low-riding vessel often used to smuggle massive quantities of narcotics across the ocean nearly undetected.

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But not this time. The U.S. Navy maritime patrol aircraft found the 40-foot “narco-submarine” more than 200 miles south of Mexico.

“Our success intercepting this drug-laden, self-propelled semi-submersible is a testament to the collaboration of our partner agencies, and demonstrates the importance of our increased presence in the Western Hemisphere,” said Vice Adm. Charles W. Ray, commander, Pacific Area. “Every interception of these semi-submersibles disrupts transnational organized crime networks and helps increase security and stability in the Western Hemisphere.”

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After removing the majority of cocaine from the vessel, the Coast Guard Cutter Station crew left 4,000 pounds of cocaine on board to stabilize it as they towed it to shore for evidence. However, the submersible began taking on water and sank along with the 2 tons of cocaine.

The same Bay Area crew has intercepted 15 different drug smuggling attempts since April and have seized more than 33,000 pounds of cocaine worth over $540 million since May 2015.

Nearly 80 percent of drugs smuggled into the U.S. in 2012 came from maritime routes, according to a U.S. Foreign Military Studies Office report. And about 30 percent of the drugs that arrived by sea come from narco submarines.

There have been 25 known semi-submersible busts in the Eastern Pacific Ocean since November 2006.