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The U.S. Coast Guard intercepted a semi-submersible vessel carrying more than 16,000 pounds of cocaine in the Pacific Ocean in the largest bust of such a vessel in the agency's history.

The cocaine seized was worth more than $181 million. The Northern California-based Coast Guard crew also apprehended four suspected smugglers after a Navy aircraft detected the 40-foot, self-propelled vessel traveling approximately 200 miles south of Mexico last month.

A statement by the Coast Guard said there have been 25 known semi-submersible interdictions since 2006. It adds:

"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."

The crew of the cutter Stratton removed 12,000 pounds of cocaine from the drug boat during the July 18 bust. Crew members left 4,000 pounds of the narcotics on board to stabilize the semi-submersible as they attempted to tow it to shore.

That effort proved futile when the vessel sank and 4,000 pounds of cocaine was lost and is unrecoverable, according to the Coast Guard statement.

The Los Angeles Times reports that:

"The smuggling vessels act like low-tech submarines, which are mostly submerged with only a cockpit and an exhaust pipe above the surface. They're extremely difficult to detect and dangerous to operate, Coast Guard officials say."

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