The US Coast Guard seized a semi-submersible "narco sub" in August 2015 that was carrying more than 12,000 pounds of cocaine. (Photo via EPA/US Coast Guard)

US authorities nearly caught a white whale this week: A semi-submersible "narco sub" carrying around 5.5 tons cocaine worth nearly $194 million. But before the Americans could haul their catch ashore, the boat sank to the bottom of the ocean with all of its illicit cargo still aboard.

Agents from US Customs and Border Protection (CBP) intercepted the drug boat on Thursday after tracking it for nearly three weeks with help from Joint Interagency Task Force South, a US military and law enforcement task force that fights smuggling on the high seas.

According to a CBP statement about the operation, the vessel was first spotted somewhere in the eastern Pacific Ocean on March 2 by agents in a P-3 Long Range Tracker, a surveillance aircraft with special sensors and radar to detect semi-submersible subs, which float just below the surface and are often camouflaged to blend in with the water.

Four suspects who were driving the boat were arrested, but CBP said the craft became "unstable" during the operation and sank — along with around 12,800 pounds of cocaine that was stuffed inside its hull. CBP did not say where the boat was when it sank. Despite losing the ship and the drugs, CBP declared the operation successful.

"This type of cooperation and teamwork produces these kinds of results where suspects are arrested and narcotics prevented from reaching US shores," said John Wassong, director of the National Air Security Operations Center at Corpus Christi. "Our crews will take every opportunity to disrupt this type of transnational criminal activity."

This isn't the first time US authorities have stopped a coke-laden semi-submersible only to lose the evidence while towing it to shore. Last year, the US Coast Guard identified and intercepted a narco-sub about 200 miles south of Mexico carrying about 16,000 pounds of cocaine. They were able to unload about 12,000 pounds of the white powder before the vessel starting taking on water and sank. The Coast Guard released video footage of the operation, which was the largest narco-sub seizure to date.

A report by the US Foreign Military Studies Office (FMSO) found that, in 2012, 80 percent of illicit drugs smuggled into the US came via maritime routes. Of the drugs that arrived on US shores by sea, 30 percent were found to have been smuggled in narco-subs.

The vessels, which are usually built in Colombia, are designed to travel just below the waterline, with only an exhaust pipe, a small wheelhouse, and and an air stack sticking out above the water. The crews are generally small, with just a captain, a navigator, and a guard to supervise the drugs.

"Typically crews are made up of an experienced sailor, the so-called "captain" who can also be the person who handles communication with the 'base' or the owners of the shipment,"Javier Guerrero, a University of Edinburgh researcher interested in technological innovations in the illegal drug trade, told Motherboard last year. "Most likely the crew is made up of experienced sailors, and the degree of knowledge and experience and their relationship with the owners of the shipment or the contractor determines the hierarchy inside the narco sub."

In the 2015 fiscal year, US authorities intercepted a total of 213,000 pounds of cocaine in the "transit zone" — an area of 42 million square miles that includes 41 nations, the Pacific Ocean, the Gulf of Mexico, the Caribbean, and seaboard areas of the US.

The semi-submersible bust on Thursday was second high-profile cocaine seizure in less than a week. Last Friday, a JetBlue flight attendant ditched two bags filled with nearly 70 pounds of cocaine after she was selected for random screening at the Los Angeles International Airport. The woman kicked off her Gucci heels, dashed out of the airport, and remained on the lam until Wednesday, when she was arrested in New York City. She now faces federal drug charges, but claims she wasn't "fully aware" of what was in her bags.