The deep sea - which gets darker till no more sunlight penetrates at about a kilometre depth and ever colder closer to the bottom of the ocean - covers most of the planet and is, thus, by far the largest habitat on earth, yet, it has been explored less than space.
Life on the edge of a frozen sea is tough. Pack-ice at both poles is constantly on the move and, in winter, freezes solid with air temperatures 70 degrees below freezing. Only in spring, with the retreating ice and light reaching the water, does life begin again. Plankton blooms and feeds vast hordes of migrating fish, birds, whales, seals and polar bears. Walruses rake the seabed for clams. Minke and humpback whales gorge themselves on gigantic swarms of krill. But it is a brief indulgence, for the ice soon returns and pushes life back into the ocean.
The boundary between land and sea is an exciting place, with animals constantly coming and going. From the open oceans, millions of seabirds are forced to come onto land to breed. Sea eagles steal kittiwake chicks from their nesting ledges. Turtles lay their eggs in the sand and marine mammals haul themselves out to fight on the beaches. Sea lions emerge from the kelp to give birth, while killer whales come crashing into n the surf to snatch the sea lions' young.
Sir David Attenborough discovers the curiosities that have led to accusations of forgery but have ultimately helped us rethink evolution. The platypus and the midwife toad are the two most famous bizarre creatures thought to be a hoax.
Tides govern marine life. Tidal marshes are some of the most productive parts of the world. Numerous plants support numerous animals, yet life is not easy; predators are attracted to these enormous quantities of food, forcing animals to seek constant protection from attack. Relief comes with the crashing waves, as the tides flow once more. Between the tides, when the sands become depleted of food and air, the worms, clams and shrimps just endure the expected pause.