Trident Missiles, and the Ohio class submarines that carry them.
undetectable, can fire up to 16(probly more) ballistic missiles with multiple independently targeted warheads from anyhwere in the oceans(including under thinnish ice) to anyhwere on earth. the only limit the subs have to staying underwater is food for the crew(not air, not water).
missile silos can be bombed, planes can be shot down, these subs cannot be touched.
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