This programme concentrates on the marine environment. Attenborough goes underwater himself to observe the ocean's life forms and comment on them at first hand.
He states that those that live on the sea bed are even more varied than land inhabitants.
Much sea life is microscopic, and such creatures make up part of the marine plankton.
Some animals are filter feeders and examples include the manta ray, the basking shark and the largest, the whale shark.
Bony fish with their swim bladders and manoeuvrable fins dominate the seas, and the tuna is hailed as the fastest hunter, but the superiority of these types of fish did not go unchallenged: mammals are also an important component of ocean life.
Killer Whales, dolphins, narwhals and Humpback Whales are shown, as well as a school of beluga whales, which congregate annually in a bay in the Canadian Arctic — for reasons unknown. Marine habitats can be just as diverse as those on dry land.
Attenborough surmises that the coral reef, with its richness of life, is the water equivalent of the jungle.
Where the breezes of the Gulf Stream meet those of the Arctic, the resulting currents churn up nutrients, which lead to vegetation, the fish that eat it, and others that eat them.
Attenborough remarks that it is man who has been most responsible for changing ocean environments by fishing relentlessly, but in doing so has also created new ones for himself — and this leads to the final episode.