In a joint operation, the US Navy, US Coast Guard, and assets from the Customs and Border Protection Office of Air and Marine, intercepted a "narco-submarine" filled with more than 16,870 pounds of cocaine off the coast of El Salvador.

On July 18, a Coast Guard cutter commandeered a self-propelled, semisubmersible vessel carrying 274 bales of cocaine weighing more than eight tons, according to a press release from US Customs and Border Patrol.

The four individuals found with the illicit contraband were detained by US authorities.

Similar to a submarine, "narco-subs" are custom-built vessels used by traffickers to move large amounts of drugs. The first narco-sub was detected in 1993 and was built from wood and fiberglass, could not submerge, and could only travel at 10 mph.