The submarine USS North Carolina out of Pearl Harbor was recently awarded a prestigious Navy Unit Commendation for “exceptionally meritorious service” during a 2015-2016 deployment to the Western Pacific.

What, exactly, the highly capable Virginia-class submarine and its crew did on the covert parts of the deployment to earn the distinction — akin to a Silver Star — is classified.

Unlike surface ships, which are highly visible, submarines are intended to be undetected, and the Navy keeps much of their intelligence, surveillance and reconnaissance activities under wraps.

The award signed by Navy Secretary Ray Mabus for assigned missions from Sept. 21, 2015, to March 12 provides some vague clues. The sub and 135 crew returned March 17 from the six-month deployment.

“The personnel of USS North Carolina completed two highly successful missions vital to national security, as well as three high-priority theater anti-submarine warfare operations,” the citation states.

The 377-foot sub, which is specialized to operate in littoral, or nearshore, seas as well as in deep water, “excelled in all aspects of attack submarine operations throughout the Western Pacific,” while the crew’s efforts “fully informed decision-makers at the combatant command and fleet headquarters levels, enabled other significant maritime operations, furthered long-term intelligence collection efforts, and advanced warfighting readiness in this critical theater.”

In terms of comparison, the guided-missile submarine USS Florida received a Navy Unit Commendation in 2013 for its 2011 role — along with four other ships — in firing 221 Tomahawk missiles at Libyan targets helping protect “civilians under threat of attack during Operation Odyssey Dawn,” the Navy said. Forty-five percent of the Tomahawks were fired by the Florida, the Navy noted.

The Navy Unit Commendation, established in 1944, can be awarded to a ship, aircraft or detachment that distinguishes itself “by outstanding heroism in action against the enemy” or in noncombat service that was “outstanding when compared to other units or organizations performing similar service.”

One of the publicly acknowledged missions of the North Carolina was an anti-submarine warfare exercise with the Republic of Korea navy submarine ROKS Kim Jwa-Jin, during which the North Carolina hosted U.S. Ambassador to South Korea Mark Lippert and the commander of the Korean fleet, Vice Adm. Ki Sik Lee.

The mid-February drill followed North Korea’s detonation of a nuclear device Jan. 6, 2016, and, on Feb. 7, the firing of a long-range rocket. South Korean media reported the submarine exercise was a demonstration of joint U.S.-South Korean ability aimed at the North.

Upon return to Pearl Harbor, North Carolina skipper Cmdr. Gary Montalvo said on the sub’s Facebook page, “We just returned from the most amazing deployment with incredible success. The crew did things for our nation that are unmatched. The (North Carolina) accomplished in one deployment what many boats accomplish in several deployments.”

The North Carolina received a Battle Efficiency (Battle “E”) award in January 2016 from the U.S. Pacific Fleet Submarine Force, and in August, Montalvo was given the prestigious Vice Adm. James Bond Stockdale Leadership Award. Montalvo stepped down from command of the North Carolina on Friday.