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 - Doug Allan, Planet Earth's polar specialist, and Didier Noirot, Cousteau's front-line cameraman - come face-to-face with fighting humpback whales and two-hundred-ton feeding blue 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 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.