Watch The Blue Planet

For years, man has used the oceans as a source of life, food and riches. Now the oceans have been put under such strain that we may be about to lose this valuable asset. Deep Trouble looks at the impact man has had on the ecology of the oceans.

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.

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 schools of giant stingrays through 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.

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 shrimps 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 primary focus of this episode is the northern hemisphere's 'seasonal' waters, that is to say, the parts of the oceans that experience all four seasons. Also, there is a particular emphasis on the impact the summer phytoplankton has on the entire yearly cycle.

As the title suggests, this installment looks at the yearly cycle at both the North and South Poles. The Arctic Ocean, as we see, is heavily impacted by the presence of polar bears, who are the focal point of all life throughout the harsh winter months. The South Pole, on the other hand, is far harsher than its northern counterpart, yet as we see still has an active, year-round ecosystem.

The open ocean or 'marine deserts' is home to the largest sea predators, and this episode follows them on their hunt. After that, despite the title, there is a look at various forms of shelter that can be found, from natural and man-made flotsam, to volcanic islands dotting the massive oceans. At each 'shelter', we get a glimpse of the thriving and intricate ecosystems surrounding them.

This second installment focusses on the largest habitat on earth: the deep sea. As they plunge deeper and deeper into the ocean, away from the light and sun above, a weird world of darkness emerges: one of strange, never before seen or filmed life-forms; deep trenches that cut into the abyssal plain; and the largest geological structures on earth the mid-ocean ridges.