The final instalment concentrates on the most unexplored 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. Down in the ocean's furthest reaches, some creatures defy classification. On the sea floor, scavengers such as the spider crab bide their time, awaiting carrion from above. The volcanic mountain chain at the bottom of the Atlantic Ocean also sustains life through the bacteria that surround its sulphide vents. 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. Off the Mexican coast, a large group of sailfish encircle another shoal of bait fish. The hunters change colour as a message of their intentions, since an attack could also be fatal to others of their number. The last sequence depicts the largest animal on Earth: the blue whale, of which 300,000 once roamed the world's oceans. Now fewer than 3% remain. Planet Earth Diaries shows the search in the Bahamas for oceanic whitetip sharks.