Exploring 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 - Doug Allan, Planet Earth's polar specialist, and Didier Noirot, Cousteau's front-line cameraman - come face to face with fighting humpback whales and 200-ton feeding blue whales. Teaming up with top whale scientists, Giant Lives discovers why southern right whales possess a pair of one-ton testicles, why the arctic bowhead can live to over 200 years old and why size truly matters in the world of whales.

Humans have long wondered if the universe may harbour other intelligent life forms. But perhaps we need look no further than our oceans? Whales and dolphins, like humans, have large brains, are quick to learn new behaviours and use a wide range of sounds to communicate with others in their society. But how close are their minds to ours? In the Bahamas, Professor Denise Herzing believes she is very close to an answer, theorising that she will be able to hold a conversation with wild dolphins in their own language within five years.

Whales and dolphins are nature's supreme vocalists, with a repertoire to put an opera singer to shame. The mighty sperm whale produces deafening clicks in its blowhole which it uses to locate giant squid two miles down in the ocean abyss, while migrating narwhals use similar sounds to pinpoint vital breathing holes in Arctic ice-floes. The pink boto dolphin creates bat-like ultrasonic clicks to 'see with sound' and to catch fish in the murky waters of the Amazon River, and also uses whistles and chirps for social conversations. Killer whales in the North Sea use wolf-like howls to round up the herring shoals which they feed on, and they and other dolphins also use percussive tail slaps and splashing leaps to signal to each other. One group of bottlenose dolphins in Brazil has even learned to communicate with fishermen in a unique partnership. But the most famous and mysterious voice of all surely belongs to male humpback whales, whose haunting operatic performances may last several hours and seem to be about singing purely for the sheer pleasure of making music.