David Attenborough examines the borders between land and water, and the life that thrives there - from the sticky mud of an English estuary and its bird and insect life to the more numerous life in the tropical mangrove forests.

He looks at the fish that hunt out of water, the world's largest reptile, the turtles that come ashore to breed in Costa Rica and the seven-foot-long leatherback turtle that can be found in Malaysia.

David Attenborough explores the ocean, which covers 70 per cent of the Earth's surface. It contains extraordinary creatures like the leafy sea dragon, the narwhal, the 'unicorn of the sea', and the whale shark. It has kelp forests, coral reef jungles, the cold black permanent night of the deep sea and affects our oxygen and weather.