Most are more than 50 feet long and built by drug smuggling groups sometimes in collusion with the Revolutionary Armed Forces of Colombia, or FARC, which funds its four-decade-old Marxist insurgency with the drug trade.
Colombia exports 600 tonnes of cocaine per year, according to United Nations monitors, about a third of it from the Pacific coast.
"ANYTHING COULD BE ABOARD"
Vice Admiral Edgar Cely worries that the FARC could use the craft to transport arms and explosives in an attack on a port. "Anything could be aboard those things," Cely told Reuters in his office at the Defence Ministry in Bogota.
The craft are constructed inland on ramps to keep them off the wet ground of the mangroves that line the Pacific coast. Overhead vegetation hides the construction sites from the air.
Painted blue to blend in with the water, they are loaded with their illegal cargo and taken by river at high tide to the ocean.
It is much more difficult to build and hide them on the heavily populated Caribbean coast, which is benefiting from a tourism boom under U.S.-backed security policies that have made many parts of Colombia safer.
Eight were found on the Pacific coast last year and one on the Caribbean, the Navy says, their hulls fitted with lead panels or water tanks to submerge them.
Authorities say they are bracing for the day when smugglers figure out how to make full-fledged submarines capable of diving deep and navigating even more quietly than the current generation, which known locally as "semi-submersibles."
Meanwhile, Cely said he is increasing land patrols along the coast to find the vessels where they are most likely to be detected, on their construction ramps.
"They are evolving quickly in terms of technology. They are getting bigger, faster and are outfitted with GPS navigation systems and satellite telephones," Cely said.
"We really have to stay on top of this."
