It almost sounds like something out of a certain 1980s spy thriller: China, according to state media, recently tested a new magnetic propulsion system that could make its military submarines much quieter and harder for the US Navy to detect.

But engineers in other countries have been working on magnetic ship drives for decades, without much to show for it. There's no guarantee Chinese engineers will succeed where others have failed.

If the magnetic drive proves reliable and Beijing succeeds in installing it in a large number of subs, the Chinese fleet could begin to match the American fleet underwater. And that could partially negate one of the United States' biggest military advantages over China. The US and Chinese navies each possess around 50 attack submarines, but America's undersea boats are much stealthier than are the less sophisticated Chinese boats.