We can survive for weeks without food, but only days without water: it is the essential element of life. Yet many millions of us live in parched deserts around the world. In the second episode of Human Planet, we discover how the eternal quest for water brings huge challenges - and ingenious solutions - in the driest places on Earth. Battling through a sand storm in Mali, Mamadou must get his cows to a remote lake but desert elephants have arrived first. Can he find a safe way through the elephant blockade? Alone for weeks on end, Tubu women and children navigate the endless dunes of the Sahara. How does young Shede know where to find the last oasis, three days walk across the sea of sand? At the height of the drought we witness a spectacular frenzy: two thousand men rushing into Antogo Lake to catch the fish trapped by the evaporating water. When the rain finally arrives in the desert it's a time for flowering and jubilation - and love. The Wodaabe men of Niger put on make-up for an intoxicating courtship dance and beauty contest.

Many consider it the greatest team ever assembled, in any sport. Yet even that description fails to give it proper credit for its impact. The original the Dream Team, the first U.S. Olympic team to include NBA stars was a phenomenon on and off the court. It mattered not that it dominated the Olympic competition. It won the gold medal at the 1992 Olympics in Barcelona, beating its eight opponents by an average of 44 points. What was important was that the Dream Team, gave fans a glimpse of basketball at its finest, and an entire world responded. The film is the definitive documentary of the historic 1992 USA Basketball team, featuring recent interviews with all 12 members, including Michael Jordan, Magic Johnson, Larry Bird, and Charles Barkley. Includes rare footage from USA Basketball's film library, and the untold story behind what has become part of basketball legend. Since 1992, basketball has exploded in popularity around the globe, rising to a place where it truly challenges soccer's status as the world's most popular sport. Much of that is due to the impact of the Dream Team, which attracted fans and followers wherever it went. "It was," said Coach Chuck Daly, "like Elvis and the Beatles put together. Traveling with the Dream Team was like traveling with 12 rock stars. That's all I can compare it to." And what a band it was. Start with three of the game's all-time greats: Michael Jordan, Magic Johnson and Larry Bird. Add the irrepressible Charles Barkley for a little spice, as well as seven more NBA All-Stars: centers David Robinson and Patrick Ewing, power forward Karl Malone, swingmen Scottie Pippen, Chris Mullin and Clyde Drexler and point guard John Stockton. Complete the roster with Christian Laettner, the best college player of 1991-92 and a future NBA All-Star. Opponents didn't have a chance, but they didn't care.

Al-Khalili travels to northern Syria to discover how, a thousand years ago, the great astronomer and mathematician Al-Biruni estimated the size of the earth to within a few hundred miles of the correct figure. He discovers how medieval Islamic scholars helped turn the magical and occult practice of alchemy into modern chemistry. In Cairo, he tells the story of the extraordinary physicist Ibn al-Haytham, who helped establish the modern science of optics and proved one of the most fundamental principles in physics - that light travels in straight lines. Prof Al-Khalili argues that these scholars are among the first people to insist that all scientific theories are backed up by careful experimental observation, bringing a rigour to science that didn't really exist before.

Professor Diarmaid MacCulloch - one of the world's leading historians - reveals the origins of Christianity and explores what it means to be a Christian. In the first episode, he overturns the familiar story that it all began when the apostle Paul took Christianity from Jerusalem to Rome. Instead, he shows that the true origins of Christianity lie further east, and that at one point it was poised to triumph in Asia, maybe even in China. The headquarters of Christianity might well have been Baghdad not Rome, and if that had happened then Western Christianity would have been very different.

Grasslands feed the world. Over thousands of years, we humans have learned to grow grains on the grasslands and domesticate the creatures that live there. Our success has propelled our population to almost seven billion people. But this episode reveals that, even today, life in the 'Garden of Eden' isn't always rosy. We walk with the Dorobo people of Kenya as they bravely attempt to scare off a pride of hungry lions from their freshly caught kill. We gallop across the Steppe with extraordinary Mongolian horsemen who were 'born in the saddle'. And in a perfect partnership with nature built up over generations Maasai children must literally talk to the birds! The honeyguide leads them to find sweet treats, but they'll have to repay the favour.

The great flood in the Okavango turns 4,000 square miles of arid plains into a beautiful wetland. Elephant mothers guide their families on an epic trek across the harsh Kalahari Desert towards it, siphoning fresh water from stagnant pools and facing hungry lions. Hippos battle for territory, as the magical water draws in thousands of buffalo and birds, and vast clouds of dragonflies. Will the young elephant calves survive to reach this grassland paradise? The experienced mother elephants time their arrival at the delta to coincide with the lush grass produced by the great flood.
In a TV first, the programme shows the way they use their trunks to siphon clean water from the surface layers of a stagnant pool, while avoiding stirring up the muddy sediment on the bottom with their feet. Lechwe swamp deer, zebras, giraffes, crocodiles and numerous fish and thousands of birds arrive in the delta. And, in a phenomenon never before filmed in the Okavango, thousands of dragonflies appear - seemingly from nowhere - within minutes of the flood arrival, mating and laying eggs. As the flood finally reaches its peak, elephants and buffalo, near the end of their epic trek across the desert, face the final gauntlet of a hungry pride of lions. In a heart-wrenching sequence, a baby elephant is brought down by a lion in broad daylight.