Montezuma Dan Snow journeys to the ancient heart of Mexico in search of the lost civilisation of the Aztecs and their last and greatest ruler, Montezuma II (1502-1520). Montezuma inherited an empire of five million people, stretching from present-day Mexico to Nicaragua, from his uncle. His rule was marked by incessant warfare. Enemy states were growing more powerful and conquered tribes were becoming more rebellious. Within months of taking the throne in 1502, he went changed a man of good reason into a pitiless autocrat who declared himself a god, believing that fear and ruthlessness were the only ways to stop the empire falling apart. Yet it was at the hands of Cortes and the Spanish conquistadors that Montezuma met his downfall. But what was his relationship with Cortes, and why did such a ruthless leader submit to his captors with such relative ease? As Dan Snow visits the ruins and picks through current excavations, he pieces together the evidence of a gripping story: a divine tragedy of errors, the clash of civilisations, the end of a world - and a very human God.

With much of it covered in ice all year round and with no daylight from October to March, the Arctic Ocean is one of the world's most remarkable oceans. Its home to a multitude of unique life forms, all highly adapted to cope with the extreme and seasonal conditions. The impacts of climate change are more strongly felt here than anywhere else in the world.

Dr Alice Roberts looks at our ancestors' seemingly impossible journey to Australia. Miraculously preserved footprints and very old human fossils buried in the outback suggest a mystery: that humans reached Australia almost before anywhere else. How could they have travelled so far from Africa, crossing the open sea on the way, and do it thousands of years before they made it to Europe? The evidence trail is faint and difficult to pick up, but Alice takes on the challenge. In India, new discoveries among the debris of a super volcano hint that our species started the journey much earlier than previously thought, while in Malaysia, genetics points to an ancient trail still detectable in the DNA of tribes today. Alice travels deep into the Asian rainforests in search of the first cavemen of Borneo and tests out a Stone Age raft to see whether sea travel would have been possible thousands of years ago, before coming to a powerful conclusion.

The world is affected by an obesity epidemic, but why is it that not everyone is succumbing? Medical science has been obsessed with this subject and is coming up with some unexpected answers. As it turns out, it is not all about exercise and diet. At the centre of this programme is a controversial overeating experiment that aims to identify exactly what it is about some people that makes it hard for them to bulk up.

Could we be unique in the universe or is there another planet similar to earth somewhere in the cosmos? Is it possible that Alpha Centauri, our nearest star, is home to another earth-like planet? Earth sized planets have been hard to find, but indirect methods are coming on line to give scientists a good survey of how many such bodies may be in the universe. How rare would it be to find life on another earth-like planet?

Blue whales are the largest animals to ever live on earth, yet they are difficult to find or track. Even some of the locations where they birth their young are great secrets. Join some of the world's eminent blue whale scientists as they embark on a revolutionary mission to identify and tag California blue whales as they migrate to a spot known as the Costa Rica Dome. This incredible voyage yields unforgettable new footage and insight into these amazing creatures' lives. Scientists use DNA analysis, recordings of whale songs, homing devices, and sea dives to shed light on the blue whale's breeding and birthing habits