Authorities in the Spanish region of Galicia seized a semi-submersible submarine carrying more than 3,000 kilograms (6,614 pounds) of cocaine on Sunday.
The drugs are thought to have originated from Colombia but authorities are still investigating, according to a spokesman for the Civil Guard, a military agency with policing powers.
The vessel was seized Sunday off the coast of the port of Aldan. Two of the three crew members operating the 22 meter-long (72 feet) vessel were arrested.
Both are Ecuadorian nationals, according to the Civil Guard spokesman, who said that authorities are looking for the third crew member.
“We are working on getting the submarine out of the water to check everything properly,” he added.
The submarine was detected as a result of an “international operation” that involved Spanish Customs, Spanish National Police and the Civil Guard.
Drug traffickers have been caught using submarines to transport cocaine into the United States, including twice this year – once in June and again in September.
Coast Guard Lt. Commander Stephen Brickey told CNN earlier this year that Central and South American drug cartels have increasingly used semi-submersible submarines to transport drugs into the US.
The vessels are not believed to be as common in Europe, however. They are expensive to build and must be constructed in remote areas like the jungle to avoid detection.
Once in the water, they are tough to detect without any prior intelligence or aircraft that spot them from above, Brickey said.
“They blend in,” he said. “Most of the vessel is underwater, so it’s hard to pick out.”
CNN’s Leah Asmelash and Brian Ries contributed to this report