WASHINGTON (CNN) -- The U.S. Coast Guard seized seven tons of cocaine with an estimated street value of $8.4 million in what one officer called "the most dangerous operation of my career."
The U.S. Coast Guard said it found seven tons of cocaine aboard this vessel Saturday in the eastern Pacific Ocean.
A Coast Guard team seized the drugs and arrested four suspected smugglers after boarding the stateless vessel in the eastern Pacific Ocean, about 350 miles west of Guatemala, on Saturday.
The vessel, a submarine-like ship known as a self-propelled semi-submersible, moves beneath the surface of the water but cannot submerge completely like a true submarine.
The ship's crew threw the engines abruptly into reverse when they saw the Coast Guard team boarding the vessel, in an effort to throw them into the water, the Coast Guard said in a news release.
When that failed, the alleged smugglers tried to sink their ship, but eventually complied with orders from the Coast Guard to close valves that were flooding the vessel, the agency said.
The Coast Guard has grown increasingly concerned about the use of semi-submersibles by drug smugglers.
The 59-foot vessel equipped with advanced navigation and communications systems was capable of traveling from South America to San Diego without stopping, the Coast Guard said.
"They tend to be one of a kind," Coast Guard Commandant Thad Allen told CNN in March. "They cost up to a million dollars to produce. Sometimes they are put together in pieces and then reassembled in other locations. They're very difficult to locate."
Allen believes smugglers are building the semi-subs in response to the Coast Guard's tactic of using snipers in helicopters to shoot out engines on smugglers' speedboats, he said. The submersibles' engines are beneath water level.
In response to what the Coast Guard calls the growing threat of semi-subs being used to smuggle drugs, it has lobbied Congress to outlaw the operation of stateless semi-subs on international voyages. The House of Representatives passed such a resolution this summer but the Senate has not yet taken action.
Despite the increasing use of semi-subs, Drug Enforcement Administration officials say most illegal drugs still are transported by traditional methods -- fishing boats, speedboats and airplanes.
Last year, the Coast Guard seized a record 355,000 pounds of cocaine worth an estimated $4.7 billion -- a 2 percent increase over 2006. The year included the Coast Guard's largest cocaine bust to date -- 42,845 pounds stacked in large bundles on the deck of a freighter off the coast of Panama.