Series uncovering the secrets of the intimate lives of whales and dolphins. This episode explores the intimate details of the largest animals that have ever lived on our planet - the great whales. From the balmy waters of the Indian Ocean to the freezing seas of the Arctic, two daring underwater cameramen come face to face with fighting humpback whales and 200-ton feeding blue whale.

Focusing on the intelligence of whales and dolphins. Like humans, they have large brains, are quick to learn new behaviours and use a wide range of sounds to communicate with others. However, how similar are their minds to ours? In the Bahamas, Professor Denise Herzing believes she is close to an answer, theorising that she will be able to hold a conversation with wild dolphins in their own language within five years.

Taking a look at whales and dolphins, nature's supreme vocalists. The mighty sperm whale creates deafening clicks to hunt giant squid in the ocean abyss, while narwhals use sound to locate vital breathing holes in Arctic ice-floes. The Amazon boto dolphin's ultrasonic calls help it find fish in murky waters, while killer whales in the North Sea use wolf-like howls to round up herring shoals.