Gliding stealthily through the ocean depths, attack submarines quietly shadow their quarry, ready to strike with torpedoes or missiles.
Somewhat neglected after the Cold War, they are now making a serious comeback around the world.
Militaries in Asia, Russia and the United States are aggressively stepping up the development, acquisition and deployment of the undersea craft.
That's because they have realized that even the best surface vessels and warplanes are vulnerable to anti-ship or anti-aircraft missiles, says Bryan Clark of Washington's Center for Strategic and Budgetary Assessments, an independent think-tank.
"So they are shifting to more undersea capabilities to do some of the offensive operations that they want to carry out," he said.
Nowhere is the trend more marked than in Asia, prompted by China's rapidly expanding military might.
Beijing has established a range of maritime defense capabilities and highly sophisticated anti-aircraft systems that prevent enemy vessels from nearing its coast.
China has also worked hard to build a fleet of attack submarines, and now boasts 50 diesel and five nuclear attack subs.
Australia signed a contract this year to buy 12 submarines, non-nuclear versions of the French Barracuda attack vessel.
Vietnam has taken delivery of the fifth of six submarines it bought from Russia. Japan is expected to increase its fleet from 18 to 22 diesel subs by 2018. And India, Indonesia and Malaysia are all developing their own underwater capabilities.
The US Navy is paying close attention -- and looking at its own fleet.
Admiral Harry Harris, who heads the Pacific Command, has warned about China's military buildup in the South China Sea, saying the United States needs more attack subs in the region.
And General Philip Breedlove, former head of the US European Command, sounded similar warnings about Russia's renewed attention to submarines under President Vladimir Putin.
In addition to providing a key military capability, submarines also act as intelligence gatherers, compiling data on enemy fleets and even monitoring what's happening on land.
The United States uses its undersea craft to monitor North Korea, China and Russia, experts say.
During wartime, submarines can cripple entire enemy fleets, while those equipped with cruise missiles can lurk off coasts and attack targets on land.
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