Modern science has made space travel more accessible than ever before, so much so that billionaires feel safe shooting up into the skies. But look in the opposite direction, and you’ll find a giant void of knowledge. Only 20% of Earth’s ocean floor has been mapped. Vast stretches of our seas are less understood than the surfaces of Mars and Venus. Yet that great unknown is now emerging as an untouched economic frontier. From mining firms to Big Tech, industries are queueing up to explore and exploit the deep seas. So get ready to dive into a world of mystery and wonder, learn about battles bubbling beneath the surface of the oceans, meet individuals making waves down below and revisit the myriad ways in which the sea has inspired everything from myths to the military.