BOGOTA, Colombia — Only a few years ago tales of traffickers plying the underseas world aboard cocaine-laden submarines struck anti-drug agents as a Jules Verne fantasy.

Not anymore.

Today, smugglers are moving tons of drugs towards the United States in so-called “semi-submersibles,” homemade vessels that travel just below the ocean’s surface and cover distances of up to 2,000 miles.

Because they leave tiny wakes, the crude subs are extremely difficult to detect visually or by radar. Even when they are spotted, crew members quickly sink the vessels to get rid of the evidence and avoid being prosecuted for drug trafficking.

Authorities seized 14 semi-submersibles last year, and another six have been captured this year, according to Colombian Navy Capt. Mario Rodriguez.

Most of the vessels move between Colombia and drop-off points in Mexico and Central America. But in 2006, police discovered a scuttled 33-foot-long semi-submersible off the northwest coast of Spain.

Colombian authorities now believe that up to 70 percent of the cocaine leaving the country’s Pacific coast is packed aboard semi-submersibles. U.S. Sen. Frank Lautenberg, a New Jersey Democrat, estimated that the vessels this year would ship up to 480 metric tons of cocaine.

“They went from being an urban legend to some sporadic seizures to a flurry in the last two years,” said an official at the U.S. embassy in Bogota. “Semi-submersibles are the transportation of choice for maritime drug traffickers.”

The smugglers are trading speed for stealth.

They used to prefer go-fast boats, high-speed fiberglass watercraft that can carry 2 tons of drugs and travel up to 80 miles per hour in calm seas. But those crafts leave huge wakes and anti-drug agents, using helicopters and their own racing boats, have become far more adept at spotting them.

So, the traffickers have moved underwater by making modifications to the go-fast boat design. A semi-submersible is, in essence, a go-fast boat with a fiberglass top fitted with air vents that stick out of the water.

Instead of high-speed engines, the semi-submersible is powered by a 200 or 300 horsepower diesel motor, allowing the vessel to move about 10 miles per hour. The resulting wake is so small that anti-drug agents or Coast Guard officials must get within 3,000 yards of the vessels to spot them.

Most semi-submersibles are built along the rivers, estuaries and mangrove swamps of Colombia’s Pacific coast, at a cost of between $500,000 and $1 million per vessel.

They leave Colombia above water at night, with only about 30 percent of their cargo. Once on the open sea, smugglers add more cocaine, diesel fuel, water, food and other supplies, and often position rocks in the bow and stern to partially sink the craft and maintain ballast.

Inside, the living quarters are cramped. Typically, four or five crew members sleep on mattresses and live on a diet of canned food, crackers and energy drinks. With no bathroom, they must climb on top of the vessel to do their business.

“It’s very uncomfortable,” said Rodriguez, the Colombian Navy captain.

But it’s also effective. Rodriguez estimates that perhaps as few as 25 percent of all semi-submersibles leaving Colombia are seized.

Built to be disposable, the vessels are constructed with valves allowing them to be filled with water. Once the drugs are dropped off — or if the vessels are about to be seized — crew members can sink them within a few minutes.

Rodriguez recalled a case in January when the Colombian Navy spotted a semi-submersible near the Pacific island of Gorgona. Before Navy officers could arrest them, the traffickers sunk the vessel. The evidence was lost and all Rodriguez’s men could do was pass out life jackets to the smugglers and return them to the mainland.

Frustration over similar cases in international waters prompted the U.S. Congress last year to pass the Drug Trafficking Vessel Interdiction Act. Co-sponsored by Lautenberg, the law mandates up to 15 years in prison for operators of semi-submersibles.

But just as authorities close one loophole, the drug traffickers seem to find others. Many anti-drug agents fear that smugglers will turn to full-fledged submarines, which would be even more difficult to spot.

In 2000, Colombian police found a massive, double-hulled submarine being built high in the Andes Mountains in a warehouse outside of Bogota. The 78-foot vessel, which was half built, was designed to descend to depths of more than 300 feet — to avoid sonar — as well as to travel 3,000 nautical miles and remain at sea for nearly two weeks.

"A submarine of this sophistication might be found in the world's leading navies," said John B. Brown III, then the acting DEA administrator, in a 2003 speech.

Another fear is that submarines and semi-submersibles could be used by terrorists. Adm. James Stravidis, who heads the Miami-based U.S. Southern Command, recently outlined his concerns in a military journal.

“In simple terms, if drug cartels can ship up to 10 tons of cocaine in a semi-submersible,” Stravidis wrote, “they can clearly ship or ‘rent space’ to a terrorist organization for a weapon of mass destruction.”

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