An astonishing close encounter with some of the most exotic creatures inhabiting the hidden depths. In the realm of the giant octopus, the rainbow nudibranch (sea slug), and the Scorpion fish, viewers become a fearless undersea explorers, discovering the strange and unusual partnerships these 'star wars' creatures forge to ensure their survival, and learning how this cooperation allows life in this enchanting world to flourish. Filmmaker Howard Hall guides an astonishing adventure. Johnny Depp and Kate Winslet narrate as Green Sea Turtles gather so Surgeonfish can strip harmful algae from their shells. A Humboldt Squid changes color four times per second like a flashing strobe light. A Mantis Shrimp’s claws have the speed of a bullet in battling a hungry octopus. Brace yourself to be submerged in a wondrous new dimension.

Professor Jim Al-Khalili investigates one of the most important concepts in the world today - information. He discovers how we harnessed the power of symbols, everything from the first alphabet to the electric telegraph through to the modern digital age. But on this journey he learns that information isn't just about human communication, it's woven very profoundly into the fabric of reality.

Horizon discovers what your cat really gets up to when it leaves the cat flap. In a groundbreaking experiment, 50 cats from a village in Surrey are tagged with GPS collars and their every movement is recorded, day and night, as they hunt in our backyards and patrol the garden fences and hedgerows. Cats are fitted with specially developed cat-cams which reveal their unique view of our world. You may think you understand your pet, but their secret life is more surprising than we thought.

Idolized in ancient Norse mythology as the protector god, Thor battles evil ogres and a giant serpent bent on destroying mankind. It's the quintessential fight for survival that culminates in a final showdown between good and evil. But was the rise and fall of the hero Thor ancient code for something greater? Today, new discoveries unlock the hidden truth behind the Norse god of Thunder. Able to wield thunder from his hammer, Thor was the most revered of the Norse gods, from fending humanity from the giants that brought calamity, to his pursuit of the Midgard Serpent. Though, his fate is told to sealed during Ragnarok, it is actually the invasion of Christianity that proves far more damning to this mighty god.

When ancient Greece fell into decline, mathematical progress stagnated as Europe entered the Dark Ages, but in the East mathematics reached new heights. In the second episode, Du Sautoy explores how maths helped build imperial China and discovers how the symbol for the number zero was invented in India. He also looks at the Middle Eastern invention of algebra and how mathematicians such as Fibonacci spread Eastern knowledge to the West.

This episode explores what can give brains a boost. In America, Angela tries out a new treatment that's proven to help memory and concentration. In Japan, a remarkable 100-year-old reveals the colourful foods that keep minds more active. Plus Chris discovers the best exercise we can do for our brains. At the cutting-edge of science, discover how injections of young people's blood may help beat dementia.