20 years ago, two astronomers made a remarkable discovery, one which would change the way we view the universe for ever. A planet outside our solar system, orbiting a distant star - an exoplanet. Since then, we have found worlds where it rains diamonds, ones that boil at 3,000 degrees centigrade and even a world with four suns in its sky. But the big question is - will we ever find another Earth? As we close in on the discovery of the 2,000th planet outside our solar system, or exoplanet, we investigates the techniques that are revealing so much about these alien worlds. The documentary asks if we are really any closer to finding another world like our own - a second Earth.

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.

Hawking is the extraordinary story of the planet's most famous living scientist, told for the first time in his own words and by those closest to him. Made with unique access to Hawking's private life, this is an intimate and moving journey into Stephen's world, both past and present. An inspirational portrait of an iconic figure, Hawking relates his incredible personal journey from boyhood under-achiever, to PhD genius, to being diagnosed with Motor Neuron Disease and given just two years to live. Despite the constant threat of death, Hawking manages to make many remarkable scientific discoveries and rises to fame and super-stardom. Hawking - a remarkable man, and a remarkable movie.

Muhammad Ali was arguably the greatest boxer of the 20th century, and that rare fighter whose presence in the culture of the day was equal to (or perhaps even greater than) his remarkable skills in the ring. Ali was a superb fighter who squared off against a variety of gifted athletes during his career, and filmmaker Pete McCormack offers a new perspective on Ali through this documentary, in which some of his opponents speak about their experiences with the man. Facing Ali includes interviews with Joe Frazier, who speaks of the bitterness that still remains of his rivalry with Ali; George Chuvalo, a Canadian fighter who recalls that while Ali defeated him, he went dancing with his wife afterward while the winner went to the hospital; George Foreman, whose memories include a powerful punch that Ali chose not to land; Leon Spinks, whose 1978 upset victory over Ali thrust him into a level of fame he was not prepared to handle; Ernie Terrell, who discusses the technique and strategy behind Ali's approach; and many more.

We all project images of ourselves online - a curated identity that defines how we want to be perceived. But what temptations exist to embellish and what are the dark implications? In this episode, enter the world of Cyberbanging, where inner city youth are using social media to brag, conspire and incite violence; two African-American women bloggers who have turned the table on the trolls who threaten them; and the creator of Abdullah X, an online character who's trying to keep Muslim kids from falling under the spell of Islamic extremists.

Nearly half a million years ago, the most advanced human yet roams Europe. Strong and powerful, Homo heidelbergensis are fierce hunters, use sophisticated tools and live in close-knit family groups. Over 200,000 years they become split into two populations by extremes of weather and environment and evolve separately into two very different species. In the North are the Neanderthals, whose physical power and resilience is the key to surviving in ice age Northern Europe. About 140,000 years ago, Africa is in the grip of a devastating drought, and something remarkable has happened to the descendants of heidelbergensis who live there. The combination of environment and chance has bred in them a unique ability that will change the course of human history. It will be this small band of southern survivors, perhaps numbering just a few tens of thousands, who will come to dominate the world and be known as Homo sapiens.