The deep ocean plays a vital role in the climate system and for biodiversity on Earth. It is pacing climate change through storage and cycling of heat, carbon and greenhouse gases. It is home to a plethora of organisms and ecosystems providing diverse functions and services. Far from being a quiescent, buffered system, it may respond quickly and in complex ways to powerful environmental and human perturbations over different spatial and temporal scales. Consequences of physical and biogeochemical changes and in the re-structuring of deep-sea biological communities and their function remain poorly observed. Ocean parameters and their variability below 2000 m are collected or inferred globally through a sparse set of oceanographic transects, from isolated moorings, and satellites measuring a limited set of integrated properties. We must bridge fundamental gaps in our understanding of the deep ocean, uncertainties regarding its spatial and temporal heterogeneities, as well as its role-in and responses to- climate change and human activities. Contributions are encouraged that describe advances in our knowledge of the deep ocean, present emerging observation capabilities and technologies, quantify requirements, lay out design strategies for deep-ocean observations of climate quality, climate-ecosystem interaction, or highlight current uncertainties through model or data assimilation-based studies.