For all of human history, the deep ocean has been a source of fear and fascination, an unknowable realm that evokes a singular, compelling question: what’s down there? But now cutting-edge technologies are allowing scientists and explorers to discover this strange and exotic underworld: a place of soaring mountains, smouldering volcanoes and valleys 7,000 feet deeper than Everest is high. A realm long thought to be devoid of life is, in fact, a vibrant new world, home to pink gelatinous predators and shimmering creatures a hundred feet long, creatures that breathe iron and communicate through their skin, ancient animals with glass skeletons and sharks that live for half a millennium.

In The Underworld, Susan Casey traverses the globe, joining scientists and explorers on dives to the deepest places on the planet. She interviews the marine geologists, marine biologists, and oceanographers as they uncover this vast unseen realm. And she discovers the mind-blowing complexity and ecological importance of the abyssal ocean and the quadrillions of creatures who live in its depths.