In the days of the Mayas, cenotes – deep, natural pits on the Yucatan peninsula - provided the only means of obtaining drinking water. But in the mythology of this advanced civilisation, these waterholes and caves were also the entrance to Xibalba – the underworld. All deceased were obliged to pass through Xibalba and wait there until they were called into heaven. It was a place in which one made sacrifices to the gods – objects of daily life, as well as bloody, human sacrifices. To this day, the relics of these acts are still in place preserved underwater for more than a thousand years. The entrance to the underworld begins at a dirty waterhole in the middle of the Mexican jungle overgrown and barely recognisable. But immediately after entry, a hall of breath-taking dimensions and beauty is revealed. As if they were sculptures, stalactites and stalagmites lend the underwater cave an almost sacred ambience. By now, one has succumbed to the fascination emanated by the world’s largest underwater cave system. We accompany four professional research divers to Yucatan – a team of specialists, able to squeeze their bodies through crevices and holes, barely larger than their bodies and who master dives that would push even the most experienced divers to their limits. With them, we penetrate worlds only a few people have ever ventured into. We encounter the remains of human victims, prehistoric fireplaces and primeval animal skeletons and undertake a dive, which takes us from the primeval forest to the open sea. We then enter Xibalba – the place of myths and the dead. In the days of the Mayas - a voyage of no return. Today, this is one of the greatest challenges one can face as a diver – the hidden world of the underwater caves of Yucatan – all filmed in 3D.

In the last of the series, Julian Richards uncovers new information from the battle in 1066 between Viking warlord Harald Hardrada and King Harold of England that marked the end of the Viking age in Britain. Results from a nationwide genetic survey show where in Britain the Vikings left a measurable contribution.

American exchange student Amanda Knox is convicted and eventually acquitted for the 2007 death of another student in Italy. Was she a cold-blooded psychopath who brutally murdered her roommate or a naive student abroad trapped in an endless nightmare? Directors Rod Blackhurst (Tribeca Audience Award-winner Here Alone) and Brian McGinn (IDA Award-winner Chef's Table) and producer Mette Heide (Peabody Award-winner India's Daughter) explore the notorious case that made headlines around the world.

For 45 years, America was locked in the Cold War with the Soviet Union, and fear of global nuclear annihilation was constant. The end of the Cold War in 1991 was supposed to usher in a new era of peace and cooperation, but it didn’t last. Tensions between the U.S. and Russia have been simmering for years. And now, the conflict in Ukraine has pushed the relationship to the brink of full-blown crisis. VICE Founder Shane Smith met Kremlin officials and American leaders to figure out what’s really driving the new standoff between the powers, while correspondent Simon Ostrovsky reported from the front lines of the bloody war in Eastern Ukraine.

A war story unlike any ever seen. A story of what happens when one ordinary man, Time magazine war correspondent Michael Ware, transplanted into the Middle East by the reverberations of 9/11, butts into history. Ware handpicked and given a shattering video tape by the most feared, most hated terrorists on the planet to announce his arrival of the world stage sets out on an epic journey into the deepest recesses of the conflict as he seeks answers. Answers that he thinks will lead him to the Truth. The invasion of Iraq has ended, and the Americans are celebrating victory. The year is 2003. Alone and in secret one man, Abu Musab al Zarqawi is planning the real war. Step- by-step he lays out his plan in a letter to Osama bin Laden: the suicide truck bombings; the bloody horrors of the civil war; the televised beheadings. He carries out his grand design to transform the invasion into one of the most brutal conflicts our time and a catastrophe for the planet...and he does it all on camera. He gives the tape to Michael Ware. The tape sets our correspondent off on an epic voyage as he seeks answers. As he seeks what he thinks will be the Truth.

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.