The underwater world of whales and dolphins is explored. Included: the diet of the whale, Earth's largest animal; the human-like qualities of dolphins and whales (dolphins, for example, are believed to have names); their reliance on sound underwater, where visibility is poor.

Deep Thinkers Like us whales and dolphins have large brains, live in complex societies and appear capable of highly intelligent behaviour. But are these marine mammals really as smart as we would like to think? From the balmy waters of the Caribbean to the rich seas of Alaska, our underwater cameramen - Doug Allan and Didier Noirot - come face to face with ingenious bubble-netting humpback whales and cunning dolphins that take advantage of hunting stingrays. In this episode we discover just how smart whales and dolphins really are. We meet the scientist who is close to talking with dolphins, discover why bottlenose dolphins have fallen in love with their own reflection and meet the whales that have the ability to empathise with us.

Deep Thinkers Like us whales and dolphins have large brains, live in complex societies and appear capable of highly intelligent behaviour. But are these marine mammals really as smart as we would like to think? From the balmy waters of the Caribbean to the rich seas of Alaska, our underwater cameramen - Doug Allan and Didier Noirot - come face to face with ingenious bubble-netting humpback whales and cunning dolphins that take advantage of hunting stingrays. In this episode we discover just how smart whales and dolphins really are. We meet the scientist who is close to talking with dolphins, discover why bottlenose dolphins have fallen in love with their own reflection and meet the whales that have the ability to empathise with us.