Authorities on a search-and-rescue mission for two missing fishermen off the Pacific coast of Colombia instead found a semi-submersible vessel loaded with more than four tons of cocaine, officials said Wednesday. The so-called “narco sub” was intercepted during a joint operation by the Colombian and Ecuadorian navies and the Colombian Air Force, the Colombian Navy said in a news release.

Officers were conducting the search-and-rescue operation for the fishermen off the southwest coast of Colombia when they detected a suspicious vessel near the border with Ecuador. Sailors with the Colombian Navy boarded the 50-foot-long semi-submersible and discovered 205 packages “of different sizes and shapes,” which authorities said ended up testing positive for cocaine.

The navy released images of the seizure as well as video showing officers unloading bundles of the alleged narcotics.

Four people who were on board the sub were arrested and about 4.5 tons of cocaine was seized, authorities said.

“With this seizure, the entry of more than $137 million into drug trafficking organizations was prevented, as well as the marketing of more than 10 million doses on the streets of the world,” the navy said in a statement.

Naval units from Colombia and Ecuador were still searching an area spanning 120 nautical miles for the missing fishermen, who officials previously said were last seen in a canoe-type boat in the area.

The massive drug seizure comes just a few weeks after the Colombian Navy intercepted a semi-submersible loaded with more than 1,000 pounds of cocaine in the Pacific Ocean.

Semi-submersibles, which cannot go fully underwater, are popular among international drug traffickers as they can often elude detection by authorities. The vessels are sometimes seized in Colombian waters while heading to the United States, Central America and Europe.  

The Colombian Navy said it intercepted at least 20 semi-submersibles in 2023, leading to the seizure of 30 tons of cocaine and more than five tons of marijuana. 

Last May, the largest “narco sub” ever recorded in Colombia was intercepted with three tons of cocaine on board. About two months before that, officials seized a semi-submersible carrying two dead bodies and a massive haul of drugs off the coast of Colombia.

Colombia produces approximately 60% of the cocaine found in the world. A United Nations report released in September found that potential coca production in the country had risen by 24% since 2021.