Summary:

The final instalment concentrates on the least explored area of the planet—the deep ocean. It begins with a whale shark used as a shield by a shoal of bait fish to protect themselves from yellowfin tuna. Also shown is an oceanic whitetip shark trailing rainbow runners. Meanwhile, a 500-strong school of dolphins head for the Azores, where they work together to feast on scad mackerel along with a flock of shearwaters. Down in the ocean's furthest reaches, some creatures defy classification. Beneath the dark seafloor, Deep sea creatures such as the sea spider filtering on marine snow, sawtooth eel gazing upward for prey shimmering from above the surface, Dumbo octopus roaming through the dark void, Vampire squid performing a special display of defense and predation, Monkfish luring prey for necessities, and the spider crab bid their time, awaiting carrion from above along with eels and giant isopods. The volcanic mountain chain at the bottom of the Atlantic Ocean also sustains life through the bacteria that surround its sulphide vents which allows shrimps to feed. Within the Western Pacific bordering Japan lies the Dragon chimneys erupted from thermal vents where different forms of baterias thrives. In Atlantic, Squat lobsters settle near the hydrothermal vents grazing on bacterias. From the depths of Galapagos Island at the site of the Nine North, the chimneys provides energy for tube worms to grow and support settlement, but after nine months the chimneys turns cold causing the tube worms to wither and extinguished. There are thought to be around 30,000 undersea volcanoes, some of them taller than Mount Everest. Their sheer cliffs provide anchorage for several corals and sponges. Nearer the surface, the currents that surround these seamounts force nutrients up from below and thus marine life around them is abundant, which allows the Nautilus to forage freely before retreat to the depth. Thirty miles away, the shoal of squids jets upwards to the stream to hunt small fish and planktons while avoiding the pacific spotted dolphins as they use sonar to confuse their prey. The giant Mola Mola settle at reef for cleaning via butterfly fish feeding parasites and cool temperature. Ascension Island is a nesting ground for frigatebirds and green turtles. Off the Mexican coast, a large group of sailfish feed on another shoal of bait fish, changing color to signal their intentions to each other, allowing them to coordinate their attack. The last sequence depicts the largest animal on Earth—the blue whale, of which 300,000 once roamed the world's oceans with 3000 individuals roam the planet which they are now fewer than 3% remain.

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