U.S. authorities have seized a submarine-like vessel loaded with more than eight tons of cocaine off the coast of El Salvador.

U.S. Navy, Customs and Border Protection and Coast Guard personnel took part in the operation last Saturday. The semi-submersible vessel was tracked in international waters off the El Salvador coast by U.S. aircraft. It was intercepted by the Coast Guard after a speedboat began to approach.

Inside the semi-submersible authorities found 274 bales packed with 16,870 pounds of cocaine with an estimated value in the hundreds of millions of dollars. Four suspected smugglers were taken into custody.

Traffickers use submarine-like vessels to move large amounts of cocaine because their low waterline profile makes them difficult to detect.

The interdiction is part of a 15-nation effort targeting international drug traffickers.