New worlds and animal behaviours are uncovered on a journey from the equator to the poles.

A groundbreaking journey to the deepest parts of the oceans.

The ingenious ways life exists in the undersea cities of tropical reefs.

This episode reveals what it takes to survive in the savage and forbidding world of the big blue, the world's greatest wilderness. We witness feats of incredible endurance, moments of high drama and extraordinary acts of heart-wrenching self-sacrifice.

It's our green seas, not the blue, that bring life to our oceans. Here sunlight powers the growth of enchanted forests of kelp, mangroves and prairies of sea grass. They are the most abundant but fiercely competitive places in the ocean to live.

On the coast, two worlds collide. Coasts are the most dynamic and challenging habitats in the ocean - that brings great rewards but also great danger. The extraordinary animals that live here must find ingenious ways to cope with two very different worlds.

In this final episode, we uncover the impact that our modern lives are having on our best-loved characters from across the series, including devoted albatross parents unwittingly feeding their chicks discarded plastic and mother dolphins potentially exposing their newborn calves to pollutants through their contaminated milk.