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. The first divers to enter that world held their breath and splashed beneath the surface, often clutching rocks to pull them down. Over centuries, they invented wooden diving bells, clumsy diving suits, and unwieldy contraptions in attempts to go deeper and stay longer. But each advance was fraught with danger, as the intruders had to survive the crushing weight of water, or the deadly physiological effects of breathing compressed air. The vertical odyssey continued when explorers squeezed into heavy steel balls dangling on cables, or slung beneath floats filled with flammable gasoline. Plunging into the narrow trenches between the tectonic plates of the Earth’s crust, they eventually reached the bottom of the ocean in the same decade that men first walked on the moon. Today, as nations scramble to exploit the resources of the ocean floor, 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 Meticulously researched and drawing extensively on unpublished sources and personal interviews, The Frontier Below is the untold story of the pioneers who had the right stuff, but were forgotten because they went in the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.