An environment where only the toughest survive, the Arctic and Antarctic are unrelenting habitats. Only in spring does life begin again. Plankton blooms and feeds fast hordes of migrating fish, walruses rake the seabed for clams, and minke and humpback whales gorge themselves on gigantic swarms of krill. But it is a brief spring feast - the ice soon returns and pushes life back into the ocean.

As the days grow longer, billions of microscopic plankton bloom under the blazing sun. Here in the temperate seas, three-ton basking sharks graze among forests of giant kelp - the fastest growing plant in the world. The forests harbor thousands of other animals, including sea otters, brilliantly colored anemones, squid, and exquisite leafy dragons. But as the year wears on, storms rage in the icy sea... a desperate challenge for the animals that remain.

Bathed in bright sunlight and warm clear water, the coral reef is a rich oasis of life - the rainforest of the sea. Bizarrely adorned harlequin shrimp carry off a starfish several times their size, while haunting songs reverberate around the reef, heralding the arrival of humpback whales. Shimmering schools of brightly colored fish battle for territory in this competitive world where you have to stand out to survive.

The sun and moon move billions of tons of water with every turn of the tide. The strongest tides empty entire bays, smash trees on riverbanks, and strand sea creatures on suddenly dry land. Weaker tides control the movements of huge numbers of fish, coaxing stingrays though astounding underwater arches. And as the water recedes, the tide can create unbelievable landscapes - like a sparkling world of salt crystals inhabited only by tiny shrimp and bright pink flamingos.

From rocky cliffs to gentle dunes, the coasts are always changing. Day in and day out, they are battered by crashing waves. Seabirds come here by the thousands to nest, while baby turtles hatch and race to the sea, pursued by hungry predators. Young sea lion pups are born and play on the sand - until a killer whale attacks, crashing in on the surf. But when breeding season is over, life returns to the sea and the shores are empty once again.