A golden era of deep-sea discovery is underway, rewriting the very notion of life on Earth and the rules of what is possible. Teeming with unsuspected life, the interconnected ecosystem deep below the waves already has a huge effect on our daily lives, influencing climate and weather systems, with the potential for much more—good or bad depending on how it is exploited. Here marine biologist Helen Scales eloquently brings to life the majesty and mystery of this alien realm, making clear the price we could pay if it is disrupted.