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The U.S. Coast Guard intercepted a homemade submarine and seized a cocaine haul worth at least $181 million off the coast of Mexico, officials said.

The bust last month is the biggest of its kind in Coast Guard history.

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The Coast Guard Cutter Stratton intercepted the 40-foot “self-propelled semi-submersible” in the Pacific Ocean 200 miles south of Mexico on July 18 and arrested four alleged smugglers, officials said. The Guard seized 275 bales of cocaine.

"Every interception of these semi-submersibles disrupts transnational organized crime networks and helps increase security and stability in the Western Hemisphere," Coast Guard Vice Admiral Charles W. Ray, commander of the Pacific area, said in a statement.

The Stratton tried to tow the semi-submersible to shore, but it sank and took 4,000 pounds of cocaine with it, the Coast Guard said. The Guard recovered 12,000 pounds of drugs before the vessel sank.

A U.S. Navy plane spotted the blue-colored semi-submersible and notified the Coast Guard, officials said. The boats have become a favorite of Colombian smugglers, who have been using them for a decade.

"All that you can see is the exhaust pipe and the cockpit from air, so they’re very difficult to see," U.S. Coast Guard Chief Warrant Officer Allyson Conroy said.

The July 18 bust is not the first time a semi-submersible has been intercepted by the Coast Guard. At least 25 such vehicles have been stopped in the Eastern Pacific since November of 2006, the Coast Guard said.

On June 16, the Stratton busted another semi-submersible carrying 5,460 pounds of cocaine, the Coast Guard said.