Two Episodes: Submarines: The most powerful military machines on earth patrol our oceans. In a secret underwater world, nuclear submarines can stay submerged for months on end, but one wrong move and a submarine can become a steel coffin. We tell the story of the Kursk submarine disaster and Russias desperate attempt to save its crew. We visit the Royal Navys Submarine Escape Training Tank, where cadets learn vital techniques in case disaster should strike. We dive with US Navys mini-sub Avalon, part of the Deep Submergence Rescue Vehicle program. And we reveal how this same technology was used in a top secret project of Naval espionage, to track a rogue nuclear submarine hidden somewhere at the bottom of the ocean. Into the Abyss: Two-thirds of our planet is underwater, yet 95% of the ocean has never been explored by man. More men have walked on the moon than visited the deepest place on earth. We go underwater with the machines that allow us to travel deeper, for longer, and more safely than ever before. They have been responsible for finding the Titanic, solving the mystery of the Challenger Space Shuttle disaster and discovering new life forms in the most hostile place on earth. Into the Abyss examines some of the deep-sea technology that has allowed us to journey to the bottom of the sea and speaks to those who have risked their lives to go there.