The Deep takes us on an epic journey into the unknown, a world of crushing pressure, brutal cold and utter darkness. It's the largest living space on our planet; and today, scientists think there’s more life here than anywhere else on Earth. This is our final frontier.

We discover alien worlds, bizarre creatures. Savage hordes of squid hunt in the depths. Coral gardens flourish in absolute darkness. A whale carcass generates a frenzy as slow-moving five metre sharks fight for their first meal in a year.

There are fish that walk instead of swim; worms that feed on bones; shrimps that spend their lives imprisoned in a cage of crystal sponge.

As we go deeper, the sheer weight of water creates almost unendurable pressures. Yet even eight kilometres down - where the basic chemistry of life was once thought impossible - we find strange new species in the dark.

At volcanic hotspots, micro-worlds blossom into life. The creatures that live here are as alien as the worlds they inhabit. Hair-covered crabs feed on gushing plumes of hydrogen sulphide. Shrimps hover on the fringes of clouds of chemicals, so hot they could melt lead...

...yet one of these geysers may even hold the secret to all life on Earth.

Coral reefs are home to a quarter of all marine species. Survival in these undersea mega-cities is a challenge with many different solutions.

A turtle heads to the reef's equivalent of a health spa - but she must use trickery to avoid the queue. A remarkable grouper uses the fish equivalent of sign language to collaborate with an octopus, flushing their prey out of hiding holes.

A metre-long, ferocious-jawed Bobbit worm hides in its tunnel. Monocle bream retaliate by squirting water to expose its sandy lair.

Filmed in super macro time-lapse, coral polyps grow and die, laying the foundation for structures we can see from space. Whirlpools, tides and caves create currents.

As darkness falls, sharks gather in their hundreds to take advantage of a grouper spawning event.

Rising temperatures have led to one of the most devastating coral bleaching event in recorded history. But there is hope. We witness one of the greatest spawning events in the oceans: corals, fish and invertebrates release a snowstorm of eggs during just one night.

The big blue is a marine desert, far from shore, kilometres deep, with almost nothing to eat - and home to some of the largest and most spectacular creatures on Earth.

With special pressure-proofed cameras, we witness record-breaking feats of endurance as sperm whales hunt for squid a kilometre down in the abyss.

Until recently, no one knew how baby turtles survived in the vast open ocean. Big Blue reveals that they head to the centre of the ocean, where they use floating debris like life-rafts.

Jelly-like Portuguese man-of-war harness sail-power to trawl for fish. Thousands of dolphins herd vast shoals of lanternfish to trap them against the surface. Unique aerial footage reveals a centuries-old legend: the savage feeding frenzy of the 'boiling seas'.

Whale sharks make an epic voyage across the Pacific to a spot where, scientists now think, it may be safest to give birth. And a pair of ageing Albatrosses give their lives to raise their very last chick.

Yet even thousands of kilometres from land, there is evidence of human activity. Globe-spanning currents carry plastic into the heart of every ocean, often with tragic consequences. Could plastic be one challenge too far for the creatures of the Big Blue?

In our green seas, competition is fierce. Sunlight powers vast kelp forests, mangroves, prairies of sea grass and blooms of algae. Here animals must fight for space and food.

In kelp forests, a common octopus must become the ultimate escape artist to avoid its nemesis, the pyjama shark; and a Garibaldi Damselfish defends its seaweed garden from marauding sea urchins - but sea otters prove an unlikely ally

On prairies of sea grass, tiger sharks play a game of cat-and-mouse with green turtles. This keeps the sea grass healthy.

In shrubby meadows of seaweed, a vast army of spider crabs emerges from the depths for an annual mass moulting, while trying to avoid predation from monster, up to four-metre-long stingrays. And male giant cuttlefish compete for a mate... while smaller male wins by pretending to be female himself.

In the mangroves, a mantis shrimp abandons his mate of possibly 20 years for a larger female. Elsewhere, dolphins, whales, sea lions and birds race to a feast - vast shoals of plankton eating fish attracted by a bloom of microscopic algae.

At the coast, two worlds collide. Coasts is the story of how our Blue Planet's wildlife survives in this ever changing world. It's a roller-coaster ride of heart-stopping action and epic drama, with characters from beautiful to bizarre.

60kg tuna are far too fast for sea lions to catch. But the sea lions herd their prey as a team. Trapped in the shallows, the fish are easy pickings.

Land crabs leap from rock to rock, desperately avoiding the water. Moray Eels launch themselves from rock pools to attack; then octopuses... Both will crawl across dry rock in search of prey.

After a 60 mile return flight, a puffin returns with one precious beak-full of food. But now returning puffins must fight dramatic duels with fast and aerobatic skuas to deliver the food to their pufflings.

In the Pacific, the bizarre leaping blenny lives above the tide-line in miniature caves. A male tries to attract a mate - but waves are a constant hindrance. These are fish that hate water!

King penguins must brave the biggest wall of blubber on the planet - thousands of gigantic elephant seals. Then they endure a month with no food as they moult.

Blue Planet II explored parts of the ocean that nobody has ever visited, encountered extraordinary animals, and discovered new insights into life beneath the waves. In Our Blue Planet, Sir David Attenborough examines the impact of human life on life in the ocean.

Albatross parents unwittingly feed their chicks plastic; mother dolphins are potentially exposing their new-born calves to pollutants through their contaminated milk; even man made noise is drowning out the natural sounds animals use to communicate... Many creatures are struggling to survive.

Despite these devastating impacts, this film contains an inspiring message of hope.

50 years ago Norwegian spring spawning herring were almost wiped out. Today, the recovered Herring stocks offer one of the greatest spectacles in the ocean as hundreds of humpback whales and orca gather for the feast.

In the Galapagos, one scientist has devoted much of his life to saving the largest fish in the sea – the Whale Shark. In the Caribbean, a community is working to save giant leatherback turtles by protecting the eggs and animals they once took as food.

But some changes in the ocean will require a global effort. Warming seas threaten vast coral reefs, and predicted rising sea levels could impact the lives of hundreds of millions of people around the world.