Semi-subs aren’t technically submarines as they’re incapable of diving.

They sit about 12 to 16 inches above water, giving them a low profile that radar can't track easily.

"There has been information that the semi-submersibles have existed for about 10 years," said Rear Admiral Joseph Nimmich, director of the Joint Interagency Task Force South in Key West, Florida.

"But 2006 was the first time US law enforcement actually saw one."

The semi-subs are between 45 to 82 feet long, travel at 8 mph and can have a range of up to 2,000 miles -- and can carry between eight to ten tons of cocaine.

"It's in fact a logical progression," Nimmich said.

"As we get better at interdiction, they move to try to counteract our success."

It is estimated that 25 to 40 semi-subs left South America loaded with cocaine in 2007, with that figure expected to double this year.

The US Coast Guard is now working with Congress to make it illegal to even be aboard an unflagged semi-sub in international waters -- whether or not cocaine is present.

Such a new crime would carry a 20-year prison sentence.

"This vessel has no purpose other than illicit trafficking," said Nimmich.

Intelligence agencies are now helping to detect the semi-subs, using all the techniques at their disposal.

"We’re using the same kinds of technologies that we used to hunt submarines in the Cold War to try and find these semi-submersibles," said US Southern Command Admiral Jim Stavridis.

Nimmich is sure that semi-subs will continue to grow in popularity, unless the authorities can find ways to stop them.

"Drug runners will continue to do this until we become more successful than they are," he said.