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Continuing their journey across the USA in one of the world's largest airships, the team questions how the atmosphere changes with altitude and how that has an impact on the life found there.

Microbiologist Dr Chris van Tulleken and former paratrooper Andy Torbet search for living microorganisms in the 'death zone' of high altitude. Parachuting from 26,000 ft, Andy has to overcome sub-freezing temperatures and fatally low levels of oxygen to undertake the experiment.

Aboard the airship, expedition leader Felicity Aston and Dr Jim McQuaid explore the impact of human behaviour upon the atmosphere, questioning whether cities can create their own weather. They unravel a mysterious increase in rainfall over the desert city of Phoenix, Arizona.

Dr Sarah Beynon paraglides for a close encounter with a hawk and a unique opportunity to study up close the adaptations that enable these birds to fly so effortlessly.

To conclude their expedition, the team scuba dive in the giant kelp forest of Monterey Bay, California, to discover how the atmosphere creates habitats below the ocean surface. One of the most fertile and diverse habitats in the seas, it is a vivid demonstration of the power of the atmosphere to reach in to every corner of the planet and make it a place for life.