No, the U.S. Navy is probably not using a multi-billion dollar submarine to listen in on your phone calls and emails on behalf of the National Security Agency.

But it could.

A long line of secretive Navy spy submarines, most recently a nuclear-powered behemoth named USS Jimmy Carter, have for decades infiltrated remote waters to gather intelligence on rival states’ militaries, insurgents and terrorists on behalf of the NSA and other agencies using a range of sophisticated devices, including special equipment for tapping undersea communications cables

Before NSA whistleblower Edward Snowden revealed the agency’s phone and Internet monitoring programs targeting U.S. and European citizens, the mainstream press paid little attention to the elusive, subsurface warship. But following Snowden’s disclosures last month, several publications including the Huffington Post and the German Der Spiegel speculated that the Jimmy Carter was aiding the NSA’s surveillance of citizens’ communications in the U.S. and Europe.

“It seems this same submarine,” the Huffington Post claimed, “was pressed into service to spy on Europe.”

The modified Seawolf-class sub, built by General Dynamics Electric Boat in Connecticut between 1998 and 2004, is almost certainly able to tap the undersea communication cables that carry much of the world’s phone and Internet traffic. But just because the warship can tap cables doesn’t mean it routinely does.

At the Navy’s request, Electric Boat inserted an extension in the middle of Jimmy Carter‘s hull that added 100 feet to its standard 350-foot length — plus nearly $1 billion to the baseline $2 billion price tag. Commander Christy Hagen, a Navy spokesperson, declined to comment on the warship’s modifications.

But Owen Cote, a submarine expert at the Massachusetts Institute of Technology, said Jimmy Carter‘s hull extension most likely contains a “moon well” — a floodable chamber to allow divers, robots and machinery to move between the sub’s interior and the water, retrieving objects off the seafloor or carrying monitoring devices and other surveillance equipment.

With this, Jimmy Carter could, in theory, tap seafloor fiber-optic cables, said Norman Polmar, a naval analyst and author who has advised the government on submarine-building strategy. “You hook something on to the cable,” Polmar said, “and come back in a month and remove the tape and take it back and analyze it.”

But underwater wiretapping is probably unnecessary. “I don’t think you need to use Jimmy Carter to do that,” Cote said. “It would be a waste of that asset.”

It’s far easier for the NSA to monitor Americans’ communications on land, Cote pointed out in an interview, with the consent of phone and Internet providers.

But it wasn’t long ago that Jimmy Carter‘s predecessor subs were involved in undersea eavesdropping — against America’s Cold War rivals. That espionage took place during a technologically simpler time, when Washington had fewer ways of listening in on communications.

“Fifty, 60 years ago, this was best method of collecting certain intelligence,” Polmar says of eavesdropping submarines. Before Jimmy Carter, there were the modified submarines Halibut, Seawolf and Parche, fitted with special equipment for monitoring and accessing objects on the seafloor, including communications cables. Parche, the last of the old breed, was decommissioned in 2004, just as Jimmy Carter was nearing completion.

The subs’ secret missions, the subjects of repeated investigations by high-profile reporters including Seymour Hersh in the New York Times, were practically the stuff of fiction.

In 1968, the Pentagon deployed Halibut to the Pacific to search for the wreckage of a sunken Soviet submarine that would later be partially recovered by a CIA team aboard a purpose-built salvage ship. Trailing a four-mile long cable rigged with cameras, Halibut found the Soviet vessel in 16,000 feet of water after just three weeks.

In the 1970s, Seawolf and Parche took risky missions penetrating the Soviet navy’s main North Atlantic bastions to tap military communication cables. The two subs sailed under the Arctic at speeds of just a few miles per hour to avoid icebergs, dodging Soviet vessels and excitable seals and walruses that might betray the U.S. ships’ locations.

The special subs placed on the cables clamp-like devices that recorded passing signals, giving Washington valuable insight into Soviet naval activities. In 1980, a former NSA employee named Ronald Pelton betrayed the subs’ operations to the Soviets in exchange for around $35,000. Pelton was arrested in 1986, tried and convicted. He remains in federal prison.

The Soviets’ discovery of the undersea wiretap alerted America’s rivals, making such missions much more difficult. “People are now aware that that’s a technological capability that we have — and that puts them on guard,” Polmar says.

The disclosure, and new technology advances, has led to an apparent shift in the spy subs’ tactics. When North Korea shelled a South Korean island base in 2010, Jimmy Carter reportedly surfaced nearby and launched a small, quiet drone spy plane to photograph the damage. Since then Jimmy Carter has undoubtedly stayed busy performing other surveillance missions and, this year, will enter a roughly yearlong period of maintenance at a shipyard in Washington State.

When the submarine returns to the fleet, it will surely resume its secret duties as America’s main underwater spy. But the special sub probably won’t be listening in on your phone and Internet conversations. Too dangerous against military rivals and unnecessary for domestic surveillance, submarine wiretaps seem to have fallen out of favor.

You’re still being spied on — just not by a submarine. Exactly what Jimmy Carter is doing is hard to say.

“I’m sure,” Cote laughed, “it’s up to no good.”

PHOTO (Top): The Sea Wolf-class attack submarine USS Jimmy Carter (SSN 23) arrives following a one-night underway in Kings Bay, Georgia, Aug. 12, 2005. Courtesy of U.S. Navy/Journalist First Class Jennifer Spinner

PHOTO (Insert A): The Sea Wolf-class attack submarine USS Jimmy Carter (SSN 23) sits moored in the Magnetic Silencing Facility at Naval Base Kitsap Bangor in Silverdale, Washington, Aug. 16, 2006. U.S. Navy photo/Mass Communication Specialist Jerry McLain

PHOTO (Insert B): General Dynamics Electric Boat moves the Seawolf-class nuclear attack submarine, Jimmy Carter (SSN 23) outdoors for the first time in Groton, Connecticut, June 3, 2004 U.S. Navy/Courtesy General Dynamics Electric Boat