We think of ourselves as a global species, but in fact we can only comfortably survive on less than a quarter of the earth's surface. In To Boldly Go, doctor of extreme medicine Kevin Fong sets out to discover how we are able to conquer the other 75 per cent. In an eye-popping series of experiments, Dr Fong uses his own body to demonstrate how unsuited our biology is to much of the planet - and how we have had to develop the technology to let us survive there.

Part 1: Down
In this first programme, about how to survive underwater, Dr Fong escapes from a sinking helicopter, walks through a tank of sharks in an antique diving suit and intentionally gives himself nitrogen narcosis. We have one fundamental problem with water - we can only survive under its surface for as long as we can make one breath of air last. But the way our bodies respond can dramatically alter how long we can make that breath last -a fact graphically demonstrated by Kevin's attempt to escape from the Royal Navy's helicopter escape trainer. As it is plunged into the water and turned upside down, the body's response is to activate its fight or flight response, preparing it for sudden action, but limiting the time he can hold his breath to just 25 seconds. It's long enough to make it back to the surface - but only just. Travelling to Egypt, Kevin meets Sara Campbell - a champion free-diver, who can hold her breath for over five minutes and dive to depths of nearly 100m. However, to really explore the depths, we need to turn to technology. In an aquarium in Cheshire, Kevin tries one of the oldest diving technologies - a 'Standard Dress' diving suit - with brass helmet and boots. These suits allowed divers to work deep underwater for long periods of time - but they came with a downside - a terrible industrial disease that saw divers coming to the surface contorted or paralysed. To demonstrate its causes, Kevin climbs into a decompression chamber to subject himself to the sort of pressure divers experience

Part 2: Up
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