How diverse is the ocean in the ``biggest'' picture? How many different ecosystems support communities (assemblages) of species uniquely adapted to those habitats? Understanding ecosystem diversity---the different kinds of systems in the marine environment, where they are located, and their geographic extent---is one of the most fundamental axes of marine conservation. Detailed knowledge of entire ecosystems potentially permits the conservation of entire suites of species, protecting not only individual species of immediate concern, but the system that supports such species, as well as co-occurring species that may now or in the future be threatened. Equally important is an understanding of how ecosystems function---how energy enters and flows through a system, and how this energy influences the diversity, abundance, and distribution of species in that system. Recent discoveries of novel ecosystem diversity and of novel ecosystem function indicate that the oceans are a vast untapped frontier for the discovery of novelty at even the level of entire ecosystems.