In the fourth episode of the series, legendary nature and cultural photographer Art Wolfe captures the great brown bear with the stunning backdrop of Alaska’s mountains and glaciers, immigrating wildebeests in the plains East Africa, and the hard-to-find gorillas of Uganda.

Iain explores man's relationship with fire. He begins by embarking on an extraordinary encounter with this terrifying force of nature - a walk right through the heart of a raging fire. Fire has long been our main source of energy and Iain shows how this meant that the planet played a crucial role in Britain's industrial revolution, whilst holding China's development back. Along the way he dives in a mysterious lake in Oregon, climbs a glacier of salt, crawls through an extraordinary cave in Iran and takes a therapeutic bath in crude oil.

During a Gurkha recruitment Palin is disturbed by Maoist insurgents, but survives to suffer as he climbs to 15,000 feet and sees the majesty of Annapurna Sanctuary. In Kathmandu he is blessed by the Nepalese king before meeting some holy men. Crossing into Tibet, he meets his first yaks at the highest monastery in the world, before heading up the Rongbuk glacier towards the summit of Everest.

Our icy adventure begins with host Dr. Iain Stewart in the Alps, where he goes down an ice crevasse to show viewers how snowfall develops into expansive masses. Then, we see how ice can shape the earth's topography giving rise to features like Yosemite National Park and Manhattan Island, in NY. Finally, journey to Greenland and see how ice sheets and glaciers are melting at a phenomenal rate that is likely to accelerate over the coming years, changing the world as we know it.

Hundreds of years ago in faraway Iceland the Vikings began to write down dozens of stories called sagas - sweeping narratives based on real people and real events. But as Oxford University's Janina Ramirez discovers, these sagas are not just great works of art, they are also priceless historical documents which bring to life the Viking world. Dr Ramirez travels across glaciers and through the lava fields of Iceland to the far north west of the country to find out about one of the most compelling of these stories - the Laxdaela Saga.

Acclaimed National Geographic photographer James Balog was once a skeptic about climate change. But through his Extreme Ice Survey, he discovers undeniable evidence of our changing planet. In Chasing Ice, Balog deploys revolutionary time-lapse cameras to capture a multi-year record of the world's changing glaciers". His hauntingly beautiful videos compress years into seconds and capture ancient mountains of ice in motion as they disappear at a breathtaking rate. Traveling with a team of young adventurers across the brutal Arctic, Balog risks his career and his well-being in pursuit of the biggest story facing humanity. As the debate polarizes America, and the intensity of natural disasters ramps up globally, Chasing Ice depicts a heroic photojournalist on a mission to deliver fragile hope to our carbon-powered planet. Directed by Jeff Orlowski