The United States Coast Guard intercepted the transfer of cocaine amounting to $181 million from a submarine 200 miles off the shore of Mexico last month. The USCGC Stratton was able to seize 12,000 pounds of cocaine from the vessel on July 18 after a U.S. Navy aircraft spotted the sub. A total of 16,000 pounds of drugs were found inside the semi-submersible vessel transported by four smugglers who were taken into custody, CNN reported. 

The Stratton tried towing the submarine to shore but the 40-foot vessel sank carrying 4,000 pounds of narcotics with it. Traffickers tend to sink ships down instead of getting their drugs confiscated. Despite that setback, the operation is still the "largest recorded semi-submersible interdiction in Coast Guard history," according to Agence France Presse.

Vice Admiral Charles W. Ray, who is in-charge of the Pacific region, said, "Every interception of these semi-submersibles disrupts transnational organized crime networks and helps increase security and stability in the Western Hemisphere." 

Mexican borders and other land-routes have been more difficult to tread since authorities have more rigorous operations that traffickers opted the sea-route. Drug traffickers prefer this kind of semi-submersible vessel because it is low-tech and difficult to detect once submerged. These ships fits drug-trafficking well since, once underwater, only the cockpit and an exhaust pipe can be seen.

U.S. Coast Guard Chief Warrant Officer stated that, "All that you can see is the exhaust pipe and the cockpit from air, so they're very difficult to see," according to NBC News. 

This was not the Coast Guard's first encounter with this kind of vessel. Since November 2006, 25 drug filled, semi-submersible ships have been captured in the Eastern Pacific. 

On June 16, a similar vessel carrying 5,460 pounds of cocaine was also intercepted by Coast Guard's Stratton.