This is the U.S. Navy's most secretive submarine

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 in 2013, 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.