Trump suggests:

'Threaten Putin with nuclear submarine patrols'

Former President says US should tell Putin it will send nuclear submarines and planes to Russia' coast if he continues to threaten Ukraine.

Watch: 3 Russian nuclear subs emerge from Arctic ice

Three Russian submarines surface from beneath ice sheet simultaneously as part of Arctic expedition.

Iran develops nuclear-powered submarines

Iran navy undergoing period of accelerated military development, with emphasis on construction of submarines; plans for aircraft carrier.