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Drug smugglers may be sinking deeper underwater to move their contraband closer to the United States.

A drug smuggling submarine, capable of holding eight tons of drugs and able to travel to the coast of Mexico, was seized by Colombian soldiers, authorities said on Monday. Last July, another fully submersible "narco-sub" was seized just by authorities in neighboring Ecuador.

Until recently, U.S. authorities had mainly seen semi-submersible vessels carrying cocaine. U.S. authorities were involved in seizing 11 semi-submersible vessels south of the border, U.S. Coast Guard spokesman Lt. Patrick Montgomery said Tuesday.

"This is more advanced," Montgomery said about the submarine seized in Colombia. "This is a new wrinkle."

With semi-submersible vessels part of the structure always remains above the surface. Montgomery said they can travel as far as 5,000 miles at speeds of up to 14 miles per hour. They cost up to $ 1 million to make, he said.

But the sub discovered Sunday can operate completely underwater, Col. Manuel Hurtado, chief of staff of Colombia's Pacific Command, told The Associated Press. He estimated it could hold eight tons of drugs.

The sub was found in a rural area of Cauca province on the Timbiqui River about 275 miles southwest of Bogota, the capital.

Hurtado said intelligence reports and tips from informants led troops to the vessel. He said the sub was empty when soldiers found it and no one was arrested.

The 99-foot-long fiberglass sub has room for a crew of six and is powered by two diesel engines and has an air-conditioned interior, Hurtado said. He said it can submerge up to 9 feet deep and is equipped with a 16½-foot periscope.

"The engines were already fully installed and ready to go," Hurtado said.

The sub had "the capacity to sail totally underwater and the ability to travel to the coast of Mexico without surfacing," he added. He said such a trip would take eight to nine days.

Hurtado estimated the vessel had taken six to eight months to build and cost about $2 million.

He said Colombia has seized at least 32 semi-submersible vessels designed to smuggle drugs in the last decade, including a dozen last year.

Montgomery, the Coast Guard spokesman, said the United States and its Latin American partners will continue to work to locate submarines used for smuggling.

"We're always adjusting just like they are," he said.

The Associated Press contributed to this report.