The deep sea is a dark, cold habitat, once thought to be inhospitable to life and uniform across its vast expanses. Technologies such as remotely operated vehicles have shown scientists that it is, in fact, home to highly diverse organisms uniquely adapted to its harsh conditions. There is still so much to learn, however, about how species and populations evolved in the deep sea. This has important conservation implications because the depletion of nearshore and shallow water species has moved fisheries increasingly into deeper waters. Rus Hoelzel discusses some of the key environmental drivers and adaptations promoting the evolution of diversity in the deep sea, with a focus on those associated with depth itself.