3/3. Author Andrew Smith takes this final journey into the submerged and often secret lives of submariners. Hidden Depths. The ocean depths are the final frontier where submariners have cloaked their activities for decades - top-secret spy missions, dangerous games of cat and mouse, and deep voyages of discovery that few could ever talk about. Now, as the Cold War under the waves recedes, the military and the scientists combine to illuminate the deep-sea abyss. Producer Mark Burman