Documentary series presented by David Attenborough which explores the planet's oceans. From the equator to the poles, new worlds and animal behaviours are revealed, from tool-using fish to mother walruses fighting for space on the ice for their tiring pups.

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.