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.