In the second programme of the series, Dr Fong is going up to find out how the body responds to altitude. The short answer to that question is: badly - as Dr Fong finds out, climbing one of the tallest mountains in the Alps, and subjecting himself to a sudden depressurization to 25,000 feet. But although we struggle to deal with altitude, the defining story of 20th-century technology was our quest to leave the surface of the earth behind and travel up through the atmosphere and into space. The natural home of our species is at sea level. Even today more than three-quarters of the world's population live at altitudes below 500m. And there is a very good reason for that - the higher we go the less oxygen there is in the air and the harder we find it to survive.