The world underwater is mind-bendingly vast and, surprisingly, as noisy if not noisier, than the world we hear above water. Aubrey discovers the importance of sound, in this dark, remote world where both visual and chemical senses play little or no role.

Aubrey, with the help of various experts, discovers the phenomenal vocal range of whales, dolphins, walrus, seals and, more surprisingly, fish.

At Bainbridge Island, near Seattle, Aubrey discovers the remarkable night choruses produced by Midshipman fish. The males produce a beautiful pure tone hum to attract females to lay eggs in their nest. Aubrey meets Andrew Bass and Midge Marchaterre, who have been studying these incredible fish for many years, and is lucky enough to sample an evening performance from these ‘humming’ fish.

Aubrey goes to America's Cornell University to discover the complexity of Humpback whale songs. He also discovers how walrus use a combination of knocks and taps to get their mate and learns about fish choruses off the western coast of Australia.

Sound underwater, it has been discovered, may also have a conservation application. Fish in Australia have been seen to be attracted back to their natal habitats to breed by recognizing the sounds of their own reefs. Experiments have shown how recordings of a reef could help draw fish to artificial reefs to start new colonies, leading fish into over-fished areas and super-stock Marine Protected Areas.

Following on from communication underwater, later in the series Aubrey learns more about the complex meaning of sounds by looking at the sounds of the natural world as well as our own language.