The deep ocean floor is the largest, most remote and therefore least studied environment on earth. In adddition to its inherent interest, as a sink for global carbon it may have an important role in modifying or responding to the predicted climatic changes of the coming decades. Until recently, the deep seas was thought to be a region of great stability and uniformity, but we now realise that it is the site of dynamic physics, chemistry and biology. The papers in this volume, presented at a Royal Society Discussion Meeting in April 1989, summarize the state-of-the-art in many of these areas and provide a starting point for future studies.