On Monday, the USS Jimmy Carter returned home to Naval Base Kitsap-Bangor, Washington flying a Jolly Roger flag beside the American flag. The Jimmy Carter is America’s most secret submarine, equipped like no other for special missions and the covert gathering of intelligence. The Jolly Roger indicated it had successfully done something. But what? We may never know.

Flying a Jolly Roger flag, black with a skull and crossbones (though it appears the bones could be cutlasses instead on this flag) is indicative of a successful mission, but, befitting its reputation, no information was released about what the USS Jimmy Carter’s latest mission might have been.

This is not the first time the USS Jimmy Carter has been spotted flying the Pirate flag upon returning to port. In April of this year, official Navy photos also show the boat returning to home port with the same flag being flown from the sail.

The use of the Jolly Roger on a submarine can be traced back to the submarines of the Royal Navy, who first used the flag in 1914 during World War I. Lieutenant Commander Max Horton commanded HMS E9 and . It was the Royal Navy’s first kill by a submarine. Remembering a derogatory comment from the officer in charge of the Navy who viewed submarines and their crews as nothing but “pirates” Horton ordered a Jolly Roger flag to be stitched together so that it could be flown upon return to homeport. Thus, a naval tradition was created.

British submarines have flown the Jolly Roger ever since upon returning from a successful war patrol, especially during World War II. Even HMS Conqueror flew the Jolly Roger upon returning to the UK in 1982 following the sinking of Argentine cruiser ARA General Belgrano during the Falklands War.

American submarines, though, have traditionally used another symbol to indicate a successful patrol: The display of a broom with the bristles pointing upward indicated a “clean sweep” had occurred. In wartime, that meant that the submarine was able to sink every target it engaged during its war patrol. USS Cheyenne flew the broom when she returned to Pearl Harbor in early 2003 following her mission to support Operation Iraqi Freedom. Cheyenne fired her entire complement of Tomahawk cruise missiles and was the first vessel to fire the missile in that operation.

USS Jimmy Carter is the last of the Seawolf-class of submarines though she is much different than her sisters. At 453 feet in length, Carter is 100 feet longer than the other two submarines of the class. This extra length is the Multi Mission Platform, or MMP. The MMP is quite unique in that it contains extra space for storage of mission critical items, such as Unmanned Underwater Vehicles (UUVs), Remotely Operated Vehicles (ROVs) and—most likely—manned submersibles as well to facilitate a variety of undersea missions such as commando insertion or deploying personnel to operate around submerged equipment the submarine is interested in.

Carter also is equipped with a set of precision thrusters that are located fore and aft on the submarine. These allow the boat to remain steady while conducting its missions, especially when operating close to shore where water currents could play havoc with such a large submarine as the Carter.

America has deployed several heavily-modified submarines that conducted these types of special missions: USS Halibut, USS Seawolf, USS Richard B. Russell, USS Parche and the NR1. The value of these types of submarines can hardly be overestimated. During the Cold War, for example, USS Parche tapped Soviet undersea cables in the Far East, placing recording devices to monitor Soviet communications in a program called .

The USS Jimmy Carter was reported to over North Korea in 2010 to assess the situation as North Korean and South Korean artillery were firing at each other. But exactly what the Jimmy Carter is doing is near impossible to say.

As a former submariner, though, I can only imagine the how pleased the sailors on board must be. While flying the Jolly Roger upon returning to port is not something entirely unique, it does have great significance, so Bravo Zulu to those guys.

Expect that significance to be a closely guarded one.