The US Coastguard has managed to lose 4,000 pounds of cocaine after making the biggest bust of its kind off California’s shores. A successful raid on a self-made submarine used to smuggle drugs last month ended with part of the cargo sinking into the abyss.

After seizing a “self-propelled semi-submersible vessel” with 16,000 pounds of cocaine on board, the authorities loaded some 12,000 pounds off it to take it to shore. The rest of the illegal cargo sunk, authorities claimed, as the crew of the Coast Guard Cutter Stratton from Alameda was towing the sub to shore.

“After removing 12,000 pounds of the narcotics aboard, the crew of Stratton attempted to tow the vessel to shore as evidence,” the Coast Guard said in a press release. “However, the semi-submersible began taking on water and sank.”

The lost illegal cargo now allegedly lies under some 13,000-feet of water.

The 40-foot vessel was first detected by US Navy aircraft more than 200-miles south of Mexico’s border, before the the Coast Guard apprehended the boat in US waters on July 18. The boat, operated by four suspected smugglers, contained approximately 275 bales of cocaine worth more than $181 million at the time of the bust.

Despite the multi-million dollar sinking fiasco, the Coast Guard praised its servicemen ability to secure US borders.

“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 amateur-made subs are extremely difficult to detect as they are mostly submerged with only a cockpit and an exhaust pipe slightly noticeable above the surface. Since November 2006 the Coast Guard has 25 known semi-submersible interdictions stationed in the Pacific. The July 18 seizure was the largest recorded such interdiction in Coast Guard history.

READ MORE: High seas: Coast Guard shows off 14 tons of captured cocaine

The crew of Cutter Stratton in particular has been successful at disrupting 15 different drug smuggling attempts since April. In June the US Coast Guard vessel has seized another similar vessel carrying 5,460 pounds. Overall, since May, the Coast Guard credits Stratton with having disrupted the smuggling attempts of more than 33,000 pounds of cocaine worth more than $540 million.