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.