Placebos come in all shapes and sizes, but they contain no active ingredient. They are the miracle pills that shouldn't really work at all. Now they are being shown to help treat pain, depression and even alleviate some of the symptoms of Parkinson's disease. It seems they're not acting alone. The placebo effect is intertwined with how is did the healthcare. Explores why they work, and how we could all benefit from the hidden power of the placebo.

1917. People have had enough of the war. Behind the scenes, uprisings are brewing, like the one that toppled the Tsar in Russia; on the front soldiers begin to mutiny as they did at Chemin des Dames in France. German submarine attacks in the Atlantic will finally pull the Americans into the war, but they arrive too late to help prevent the carnage of the Battle of Passchendaele.

Few sounds are more beautiful or moving than the underwater songs of the humpback whale. Male whales compete with their songs, which often last for 10 minutes at a time, and can be repeated for hours on end. Whales separated by thousands of miles of sea will sing almost identical songs. Researchers have found that the songs change throughout the breeding months, following a mysterious pattern repeated across the waves. Whales also use sound to hunt. To catch herring, humpback whales release a stream of bubbles to form a shimmering, circular fishing net. Emitting a repetitive loud scream, they scare the fish into a tight ball, then lunge out of the water to swallow the shoal whole. Now it seems that the long-held image of the gentle giant must change to one of a ferocious and opportunistic hunter.

The rainforest is home to more species of plants and animals than any other habitat on the planet. But for humans, life there is not as easy as it looks. Life in the trees requires great skill, ingenuity and sheer bravery. The Matis of Brazil carve 4-metre-long blow-pipes to hunt monkeys - in near total silence. Deep in the Congo forests, Tete defies death by scaling a giant tree using nothing more than a liana vine, and he must then negotiate an angry swarm of bees - all to collect honey for his family. Three children from Venezuela's Piaroa tribe venture deep into the jungle to hunt tarantulas - to toast for lunch! In West Papua the Korowai tribe show-off their engineering skills by building a high-rise home 35 metres up in the tree tops. Most memorable of all, in Brazil we join a unique monitoring flight in search an un-contacted tribe...