At least a dozen warships provide a physical guard while the technical wizardry of the world's only military superpower offers an invisible shield to detect and deter any intruders.
That is the theory. Or, rather, was the theory.
American military chiefs have been left dumbstruck by an undetected Chinese submarine popping up at the heart of a recent Pacific exercise and close to the vast U.S.S. Kitty Hawk - a 1,000ft supercarrier with 4,500 personnel on board.
By the time it surfaced the 160ft Song Class diesel-electric attack submarine is understood to have sailed within viable range for launching torpedoes or missiles at the carrier.
According to senior Nato officials the incident caused consternation in the U.S. Navy.
The Americans had no idea China's fast-growing submarine fleet had reached such a level of sophistication, or that it posed such a threat.
One Nato figure said the effect was "as big a shock as the Russians launching Sputnik" - a reference to the Soviet Union's first orbiting satellite in 1957 which marked the start of the space age.
The incident, which took place in the ocean between southern Japan and Taiwan, is a major embarrassment for the Pentagon.
The lone Chinese vessel slipped past at least a dozen other American warships which were supposed to protect the carrier from hostile aircraft or submarines.
And the rest of the costly defensive screen, which usually includes at least two U.S. submarines, was also apparently unable to detect it.