The oceans are an integral part of our lives. Their influence dominates the world's weather systems. They support an enormous range of life, from the largest whales to the smallest plankton, whilst still remaining largely unexplored and mysterious. This episode demonstrates the sheer scale, power and complexity of the 'Blue Planet'.

David Attenborough takes us on a voyage into the unknown with a look at the strange, otherworldly creatures that live in the very depths of the ocean, many of which are new to science. Terrifying fish with massive teeth that eat prey twice their size, weird flashing jellyfish, and deep sea sharks all inhabit submerged mountain ranges and volcanoes far larger than anything on land.

The open ocean covers more than 360 million square kilometres. Much of this huge expanse of seawater is marine desert with virtually no sign of life. Yet somehow life survives out here. Among others, we follow the extraordinary life of the yellowfin tuna from a tiny egg to a 200 kilogram, voracious predatory giant.

Life on the edge of a frozen sea is tough. Pack-ice at both poles is constantly on the move, and in winter freezes solid. Only in spring, does life begin again. Plankton blooms and feeds vast hordes of migrating fish, birds, whales, seals and polar bears. Walruses rake the seabed for clams. But it is a brief respite, for the ice soon returns.

Sunlight is the vital source of energy used by the countless billions of plankton which live in the world's temperate sea. Forests of giant kelp harbour thousands of animals. Sharks, sea otters, brilliantly coloured anemones, squid and exquisite leafy dragons are just a few of the other creatures that live in this cool, rich water.

David Attenborough narrates a natural history of the oceans. Developed over centuries, coral reefs house fragile ecosystems, but they too have to fight to survive.