Sea Hunter is designed to autonomously stalk the seas for up to three months at a time.
Russia's aggressive modernization program means the US needs to urgently reassess its nuclear forces.
Hewn out of a mountain, Olavsvern, Norway, is one of the most impregnable hideouts in the world.
Vice Admiral Joseph Mulloy told a congressional committee that China was building a large number of "fairly amazing submarines"
Russia is deploying troops and testing missiles in the polar region, poised to commandeer its massive energy reserves
Experts say the technology will give submarines better underwater range and speed compared to other diesel-electric vessels that use air independent propulsion (AIP) undersea.