We do not see the ocean when we look at the water that blankets more than two thirds of our planet. We only see the entrance to it. Beyond that entrance is a world hostile to humans, yet critical to our survival. And each advance to discover more about the ocean has been fraught with danger, as divers had to survive the crushing weight of water, or the deadly physiological effects of breathing compressed air. The Frontier Below recalls a story of human endeavour that took 2,000 years to travel seven miles, then investigates how we will explore the ocean in the future. Meticulously researched and drawing extensively on unpublished sources and personal interviews, The Frontier Below is the untold story of the pioneers who had the right stuff, but were forgotten because they went in the wrong direction.